Not really sure what you are seeing. Yang basically pulls straight back with his palm flat. Think paddlewheel, not propeller.

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Doing breaststroke with big paddles a lot of the forward motion actually comes from when you sweep your hands back rather than out.

Adam Peaty has a really wide pull but he is super strong: https://www.youtube.com/watch?v=Uu2IA6pnFwM You can see how much power he is generating.

He looks like he'd be strong in the weight room.

Look where his hands and forearms are pointing. They almost never point straight back towards the back of the pool. They point back relative to his body for a short period, but because the body is rotating they point out towards side of pool or in towards center of pool relative to the pool itself. But the pointing to the back of the pool is a really small fraction of the full stroke. Same way when you kick, you actually move forward without any body part pointing 100% of the back of the pool.....the leg and foot are moving mainly up and down with a final flick backward (that's what you told me to do), but the outcome is the water moves back. Water and air is a weird media. in terms of how the motion is generated relative to hard ground.

Intuitively you would think an ole miss paddleboat would generate more backwards force than a tiny prop, but it does not work that way due to visocity and drag of water on the surface that you're trying to move water with. If not you'd never generate any forward motion during the breast stroke sweep and catch.

Are you looking at the plane of the hand? Because that's what matters. The plane surface isn't really affected by rotation perpendicular to that plane,

It is possible that you are seeing things I can't, as I'm just on my iPhone right now, but I don't think so.

Kicking is a little different due to biomechanical limitations, but in essence, you are getting your propulsion from driving water directly backwards. That's why ankle flexibility is so critical and good kickers actually use a good bit of knee bend. To get that top surface of the foot as perpendicular as possible to the direction of travel to drive as much water back as possible.

edit -

just went to my laptop and watched a few strokes from the underwater cam at 0.25x speed. the entire front and middle of part of the stroke is pretty much perpendicular to the direction of travel, and he turns his palm inwards at the very tail end of the stroke, at least with his left arm. but that isn't really where the power is being generated, especially not the way Sun swims.

I just looked at a couple of videos of phelps and thorpe, and its the same thing. pretty much flat relative to the ends of the pool. you might see a little angle here or there, but that isn't what they are going for.

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I try to explain this in my teaching. Most of the stroke you aim to pull perpendicular but with the changing angle of the body we end up doing a slight "S" pull. Historically I was taught to exaggerate the S to create lift propulsion.
I have some studies on a memory stick somewhere showing that at 45 degrees a hand can generate as much propulsion as a perpendicular hand.
In short it's either complicated or simple.
These days most coaches don't teach the S pull anymore. While it may or may not increase propulsion it also increases drag.

Between 40 and 60 degrees is the magic numbers.
Depends on hand size and amount of force required to move you forward.

A good streamlined swimmer or large hands in distance mode will be around the 40 and a poorer streamlined or small handed person will be up around 60deg.
Swimmers with narrow strokes tend to have higher degrees and wider strokes smaller degrees.

The lower the angle of attack the wider you can have your fingers and effectively have bigger hands.

Just to be clear, the 40 degrees, let's say you are pulling back and thumb is "in front of pinky" relative to direction of travel, that 40 degree angle, if I draw a line along the width of the pool is it 40 degrees between the width of the pool and line going through thumb and pinky, or is it 40 degrees between direction of travel and line between thumb and pinky....to some degree it seems it does not matter since 45 degrees and 40 degrees is almost the same.

In any case, even though hand is perpendicular to body, its because of the body rotation (read streamline) that we are seeing this pitch. Flatter/wider swimmer will have "less pitch" because their hand is perpendicular relative to body and body is flatter

Degrees are relative to direction of motion of the limb.
Limbs don't go back, they out and down then in and up a little, then out and up to exit the water.
Perpendicular to motion of your body.

Smaller hands, sprinting and heavier masses need a higher angle of attack to create a solid pull, just like a aircraft taking off and landing when they use a high angle of attack and more curvature with extendable flaps for more lift and just accept the lack of efficiency that driving the wing harder than it is designed for produces due to extra drag.

Larger hands, cruising speed and lighter masses run at smaller more efficient angles of attack.

That's the double whammy that poor body position brings, because of the higher body drag, the hands need to operated at higher angles and efficiency of the pull suffers.
So not only are you bleeding energy to push the body along because of extra body drag, you are also bleeding energy at the hands due to poor drive efficiency.

If you draw out a resultant graph you will see much less energy going into producing forward motion and more into drag.

Based on what you are saying these paddles with only the thumb hole and keeping the pressure on the rest seems like they would be helpful. Maybe I should go back to using them because the hydrodynamic lift water and hand that is needed for these to stay on.