I tested a pair of Force Fins in the 1980's, when Mr. Evans gave me a pair, and have these observations (about the original Force Fins):
--For those used to regular fins, they take some getting used to. The Force Fins have power only on the down stroke; they use the up stroke (when swimming face down) as a recovery, and don't provide propulsion.
--The best way to use them is to do a powerful down kick, and really emphasize it. This gets the most out of the fin's elasticity. It is also very comfortable because of the foot pocket design.
--On the surface, people used to regular fins will tend to break the surface and splash a lot with Force Fins.
--Cinch up the strap fairly tight, because it can come off in surf (my son lost one in the small surf at Hawaii).
--They are the easiest fin to walk in so far designed.
--If you use it for a "frog" kick, modify the kick so that you don't use the bottom of the foot (or fin), but the top. The harder the kick, the better.
--Short dolphin kicks work well.
Having said that, they don't work as well for propulsion as a monofin (I was Fin Swimming Director for the Underwater Society of America when I evaluated them), but then again, no regular fin does (with perhaps one exception, but it's not commercially available).
SeaRat
PS--Does anyone have a use for one, regular, black Force Fin from the 1980's?