SEALs don’t like split fins either.

[Angelo Farina]

What you are describing sounds like the usual "modified frog kick."

Not what I have seen in videos about modified frog kick, for example here:
The guy is keeping the knees angled at 90 degrees, so the legs are higher than the cylinder, and the heels would touch the coral under the ceiling of the cave. Furthermore, for advancing, the movement only involves the last part of the leg and the ankles. In our horizontal scissor kicking the movement is at the hip, both knee and ankles are perfectly straight and not used.
Finally, the guy keeps his arms partially stretched in front (and partially below) the body line, causing a terrible drag. Inside caverns we always keep the hands along the body, grabbing the lower part of the cylinder, for minimizing drag and keeping it "low" (again, for avoiding that the cylinder touches the ceiling of cave, where delicate red coral is living protruding down).
Totally different, from what I see. I will search for a video showing my wife while penetrating a narrow tunnel... I have not it at hand.

It is not really a scissor kick because the legs do not pass each other.

When you close a scissor, the two blades end up touching each other, they do not cross!
they cross in flutter kick. I have seen vertical scissor kick practised, the diver open the legs vertically, then closes them and stays with them together, without crossing. That is scissor. Instead flutter makes the legs to go up and down, never resting together.
Coming back to topic. One of the kicks employed by Navy Seals is a mixture between flutter and scissor (vertical). They make one complete flutter (so each leg goes up and down for an entire cycle), followed continuously by one scissor (so the legs end together), and then the diver profits of inertia for advancing without kicking for 3-5 seconds. Then the whole cycle (one flutter, one scissor) is repeated, but starting with the other leg up (for avoiding to run slightly on one side if you always start with the same leg). This is more efficient for very long travels.

 

[formernuke]

One thing we have to remember. Spec ops are doing something totally different than us. They are get to there objective as quickly as possible usually without being seen, do objective and get out.

We are get down "look at the pretty fish" mosey along take in sites, take some pictures etc.

Of the splits I tried the only ones worth considering were the atomic smoke on the water as they were the only ones I could frog or helicopter in, but they did not work as well as paddles did.

 

[Steelyeyes]

I like my split fins and bought them because solid fins gave me really bad leg and toe cramps.
But my split fins have been gathering dust since I got freediving fins - I imagine I'll go back to them someday when I go on dives the long fins just can't handle.
At some point I want to try jet fins - I can frog kick and such in my splits but I want to see how jet fins work for me.

I like my OMS Slipstreams. They're fine for a mellow shore dive with no current but when I need to sprint ahead to set up a video or still of an eagle ray or something I can dig down and get some speed on. Same in the rare event of bucking a heavy current. Since I used to travel a lot I have yet to try the freediving fins but that's next.

 

[JMBL]

Split fins is not for me. Took 15 years for this idiot (me) to figure it out. How some genius to convince a lot of people to use it is just beyond my comprehension. Frogs & fish don’t have split fins.

Nor do they have suits, masks, BCD, open circuit, or rebreather for that matter.

Ok, I'm out

 

[drbill]

I'm shocked that the Navy would issue split fins. Don't they know that if you wear them you will die, die, die?

Yep, I must have died a thousand times over the years!

 

[Eric Sedletzky]

AFAIC, the most efficient body position and finning technique that yields the most forward movement for energy expended is stretching your body out straight with legs back straight and arms to sides or reaching out forward straight. Finning should be slow steady alternate standard fin kicking with knees and ankles stiff and movement originating from hips. Of all techniques I have tried this is the most power to efficiency ratio I can come up with combined with freediving fins. If you want to see the most efficient speed to energy ratio just look at free divers and what they do. They have to make the best out of every element since they are running off one lung full of air.
I don’t see them hanging like sky divers waddling around with knees bent up at 90 degrees.
I very seriously doubt the SEAL’s use a modified frog kick either, they use a standard leg back straight kick.
I think this is where a lot of the discrepancy comes from in the fin wars.

 

[Diver below 83]

Another interesting shot. This is a para rescue squad. Another interesting mix of fins. Looks like some splits and some jets.

 

[eleniel]

Made the same comparison several times, also with my wife and my two sons, and the result was always the same: nothing beats free diving fins. They are manufactured of different length and different stiffness, and the best ones have interchangeable blades, so you can set them properly for the real usage, the amount of drag you have to win, etc.
Also consider that jetfins or split fins are the same since decades, no evolution, freediving fins are instead being strongly improved year after year, in the last 20 years both materials and shapes changed incredibly, boosting the performances even further. This resulted in a huge variety of fins and blades, and you need time and tests for selecting the proper high-end freediving fins for your usages. You probably need to own at least two or three sets. And they are very expensive!
But there is no contest: when you need to swim against current, or to carry an heavy load up to surface without the help of your BCD, nothing surpasses their thrust. On the other side, they are soft and elastic, impossible to get cramps if they are of the proper length and stiffness for your legs.
Someone is against long, flat freediving fins as they make it difficult to perform frog kick, which is true. But I and my wife did use them for cave diving (at Capo Caccia, Sardinia) for more than 10 years, and we developed alternative kicking styles, which are more efficient and with less water perturbation.
The most interesting one is the horizontal scissor kick. The legs are kept perfectly straight and perfectly horizontal (not raised up, with flexed knees, with the risk to touch the ceiling of cave destroying the red coral living there).
You slowly open your legs, up to an angle of +/- 45° (that is 90° in total) keeping the fins flat (horizontal blade), so they will not create a significant reverse drag. When the legs are open wide, you close them quickly, angling the fins so that their blades are almost vertical, and they squeeze the water one against the other (the lower face of the blades will be face to face at the end of the squeeze). This pushes you forward, with the water being pushed backward (not down, not up). At this point you are launched fast, in a very hydrodynamic position with minimal drag, and you stay still taking profit of inertia.
A good horizontal scissor kick with long freediving fins can make you advance up to 5-6 meters, against the 1.5-2 meters you get with traditional frog kick and jetfins. And keeping a much more streamlined profile, as the legs are always perfectly straight, no risk to touch the bottom with your knees or the ceiling with your heels.
Of course there is also a reverse horizontal scissor kick (much less efficient, indeed) for going backwards inside a tunnel: keep the blades vertical while enlarging quickly your legs, and then close them more slowly keeping the blades horizontal.
No training agency actually suggests to use long freediving fins for cave diving, nor teaches these special kicking styles. I and my wife are a bit heretic here...

I love my freediving fins SO much and I know that exact kick you are talking about! I started using it while doing photography on my last Cozumel trip because it helped me get in close to my subject without stirring up the sand or kicking nearby coral.
Those fins make my diving so effortless, even against currents, it's amazing. Normally I dive slowly like everyone else but if something happens and I need speed, I like knowing they are capable of whatever I need to do.

 

[eleniel]

I like my OMS Slipstreams. They're fine for a mellow shore dive with no current but when I need to sprint ahead to set up a video or still of an eagle ray or something I can dig down and get some speed on. Same in the rare event of bucking a heavy current. Since I used to travel a lot I have yet to try the freediving fins but that's next.

Travel is the biggest issue for freediving fins so far imo. My current solution is to toss them in a mesh bag and carry them on as my personal item - they are obviously a bit long but they slide between the seat and the outer wall of the plane nicely (if you or a buddy have a window seat) or you can use them to take up your entire foot space (if they are the type you can disassemble, they will fit and sit fairly flat), or you can put them on top of your bags in an overhead compartment (don't put them in first! They go in last and on top.) So far I haven't had any issues traveling with them beyond the inconvenience of carrying them around.
If I can carry on a skateboard (which I can!) I can carry on freediving fins. I'm considering trying a backpack with a skateboard holder as my carryon - then I can just strap the fins to the bag until I'm on the plane.

The only drawback I've found with them is in wrecks and any caves or swimthroughs that are really tight/twisty/winding. It's really hard to not scrape them on things or kick things. I'd stick with them exclusively except they really aren't ideal for tight spaces.

 

[Angelo Farina]

The only drawback I've found with them is in wrecks and any caves or swimthroughs that are really tight/twisty/winding. It's really hard to not scrape them on things or kick things. I'd stick with them exclusively except they really aren't ideal for tight spaces.

As said, I and my wife did use freediving fins in caves, in Sardinia, for more than 10 years. But of course ours were not really the longest ones (which can be well beyond 1.20 meters), our owns are approximately 90 cm.
At the beginning we were touching with them everywhere. But it was very instructive: we did learn to always stay with completely stretched legs, perfectly horizontal (at Capo Caccia what really matters is not to damage the coral under the ceiling, the bottom of the cave has nothing which can be damaged).
We laugh when we see those so-called "cave divers" staying in this ridiculous position:

See the long fins (more or less as our ones), but the legs are bent (with more than 90° angle at the knee) and arms are stretched forward-down, instead of staying close to the body.
We did always correct our students when seeing them with bent knees and partially stretched arms...
It seems that this is how they are trained, nowadays, in "cave diving" courses. And frog kicking with bent knees, which is really terrible.
Doing this, they would destroy the red coral in Italian caves. It takes 30 years to grow again...
Here you see the typical "ceiling" of our caves: