I say "yes and no". I do agree that you create a bigger paddle with your hand, but since you are pulling more water, your heart has to feed your working muscles with more oxygen since you are creating a bigger catching surface to anchor your arm. We all know that you don't go "that much faster" using massive paddles because we generally don't have the conditioning and the rest of the technique that is needed to really use the massive paddles. Making your hand paddle bigger has the same effect on a smaller scale. 5% gain in speed is ~4-5 seconds on a 100m....that's not going to happen for anyone with some swim background. Maybe for a beginner, but 4 seconds per 100m is massive for anyone who has been swimming for a while.

This has been around for a while. The rake hand was debated when Ian Thorpe was winning everything.
So I tried it. If you can keep the correct distance between your fingers then it works but I can't maintain it for long before my fingers splay and flex. Keeping fingers together with enough tension is easier mentally.

Do I remember something from Doc Counsilman on this? If so, this has been around from the beginning of time.

Spread fingers may increase resistance at the hand and thus allow you exert more force but that's not the same thing as improved efficiency. The increased force is the result of turbulence around the fingers. This allows you exert more energy but it may not be as productive as if you spent it elsewhere. There's no way it would provide improved efficiency at the hand, whether it could provide improved efficiency for the stroke as a whole is less clear.

There is a bit of "webbing" in your palm that dissappears if you have your fingers too tight and just looking at it purely from physics, the surface area of your fingers would become smaller by holding them together. I have watched Videos of Grant Hacket entering the water "thumb in" and then when he goes to pull his thumb goes "way out" to maximize the webbing between the thumb and fore finger. I just can't keep focused long enough to keep moving thumb in and out on every stroke, so I just swim with "thumb out".

Dev

Sorry I missed your post. Agreed, there is extra resistance by increasing turbulent flow between fingers thus "better catch". This is in addition to "natural skin + flesh webbing". It is unclear what the better webbing and more "resistance due to turbulence" does for the overall efficiency of the stroke and what the percent speed gains would be. It would be impossible to measure because better catch means more oxygen utilization too so as I mentioned above, you don't necessarily go faster when you have big paddles.

Just relax your hand when swimming. If anything, squeeze your two middle fingers together and don't worry about the others. This will make your hand strong as a paddle and leave the rest of your hand and fingers relaxed. No need to over squeeze, just a bit of focus on the strength of those middle two fingers.

What would you consider not that much faster? Suppose one is much much faster (10s+/100yds for same energy), what would be the technique issue causing the difference?