Thanks for the link. That claim actually doesn't meet my "all else being equal" idea in that the ST ad links a reduction of exertion to a reduction of air consumption, but of course the fins are credited with a reduction of exertion, so the link between the fins and consumption is at least implied.
While most posts in this thread are reporting impressionistic evidence, there have been findings that split fins do in fact require less effort on the part of the diver to move a given distance in the water (based on distance and heart rate monitoring, for example, see
this report, scroll down to the SP Twin Jets section). Just because a diver doesn't notice the effort in the leg muscles doesn't mean s/he's not moving efficiently, and I think that's the yardstick people often use to assess whether they're getting what they want in terms of propulsion. The effort needed by the leg muscles is not a good indicator of how effective the kick performance is. (As an analogy, a car with a hole in the muffler might sound like it's working harder and moving the car faster, but in fact the noise is a poor indicator of performance).
My own (admittedly impressionistic and unscientific) experience in diving
against currents with any kind of fins is that it takes a lot more effort than diving
with currents, and that this causes fatigue and higher air consumption. Furthermore, when diving against currents the tendency is to use big, wide fin strokes. These kind of fin strokes just don't work well with split fins, and even against a current a diver can get very good performance with splits if s/he sticks with smaller, quicker kicks. If a diver is wearing splits and tries to conquer a current with big fin strokes, s/he will be disappointed with the resulting performance.
I saw the Rodale's review years and years ago, and it's funny that the air consumption bit didn't even register, if it was in there, but I still remember the rest. Maybe because I've never had to worry much about consumption? I suppose a buyer will consider the "main sales pitch" to be the thing s/he is most interested in.
