Ok, So I will not keep you all in suspense any longer.
Ok Luis, I'll bite.
First, the "trick" in your questions is that no one should ever "rely" on their J-valve as a safety reserve. Second, the most important item of equipment is a watch and knowledge of your own air consumption rate at the planned dive depth. Oh yeah, and a surface tank checker guage to see what your starting pressure is. Close?
Nice try, but not really the skill set I was looking for.
I hope I don't flunk, but my answer is: Good gas management and a watch.
A watch is always a nice thing to have. Waterproof watches used to be very expensive and they weren’t always reliable. I have a US Divers watch case to keep a regular watch from flooding. It was in the 50’s and some of the 60’s catalogs.
Seriously
… That NASDS gadget came out much later that the SPG. We were all probably using SPG before that thing was around.
Planning you dive and diving your plan is the most important skill, I think. You should know how far and/or how deep you can be when you are down to your last few hundred pounds.Most important equipment? Depends. A dive watch, certainly, in order to adhere to your dive plan.
If you were from NJ doing deep wreck dives, you probably did plan your dives, but in the Caribbean we just planned to go to the surface when we got low in air. Yes, we were (some of us still are) a lot more casual about all this planning.
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All very good answers, but not what I was looking for in skills.
The
first skill is to check your reserve valve frequently or at least occasionally. Similar to checking SPG. Every once in a while you were supposed to reach back and not only check the lever position, but I was taught to actuate the reserve, open it and close it back-up. That was the only way you new it would be there when you needed it.
Now for the
most important skill that was often overlooked when
you totally relied on a reserve valve: that would be
surface swimming! 
All reserve valves gave you was just enough air to get back to the surface (leisurely and safely, safety stop was not applicable). Once you got to the surface, you had to swim back home. The reserve valve warning never provided enough gas to swim home underwater.
If you had a watch and you could estimate your halfway point, sure you could try to turn back to end up close to your exit point, but that was not what a reserve valve gave you. Again a dive watch was a very expensive piece of gear and not always reliable. I had several friends whose “waterproof” dive watches flooded.
As a high-school student that worked in a dive shop my most expensive piece of gear was my dive watch (until I bought my Fenzy) and we all did many dives before we were able to afford a reliable dive watch.
When I did my original training in 1971 we had to swim pool laps with an empty tank on our back (or push it in front of us). Some of the laps we simulated that we lost our fins, then we had our fins but lost our mask, then we had our fins and mask but we lost our snorkel. Note: we didn’t have any flotation device, just an empty steel 72 which kinds of floats when empty.
So what I learned from all that training is that the most important piece of equipment if you have to swim home with an empty tank (and no flotation device) is
a snorkel!
Note: In places like California they determine that they need to safe some air to swim back under the kelp. I assume that is part of the reason why Scubapro came out with a reserve valve that could be adjusted from 300 psi reserve to 600 psi reserve. But this is a different story. Now some dive operators want us to surface with 700 psi
. We are not in California, we can swim on the surface, if needed.
I hope that was fun.
I should add, we all new how to use the decompression tables and someone in a group would have a watch.
We were normally not diving very deep and it was loosely assumed that a steel 72 could not give you enough time (in relatively shallow water, less than 80 feet) to get into deco obligation. Yes, looking back, not a very good assumption for a bunch of young relatively fit kids, but we luckily never had an issue.
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