I’ve been thinking at this question for a good hour now, but I can’t wrap my mind around the physics involved. Wouldn’t bringing your legs back before the top of the swing (to “swing” them back and keep you going) negate what you’ve gained through the precedent leg throw because of energy conservation?
In: 7
It isn’t really about your legs, it’s about shifting your centre of mass. The body lean is more important than the leg swing.
On the way down, you lean back to shift your weight and swing your legs out for a bit of a momentum boost. On the backswing, you do the same in reverse. But it doesn’t work as well in reverse, so you gain most of your swing height when you swing forward.
One thing people haven’t mentioned yet is that your exerting force on the chain. You’re right that if you were somehow just floating in space, doing the kicking motion wouldn’t do anything because of conservation of momentum. However pulling your legs back on the back swing is like pulling on the chain, which generates potential energy. Imagine hanging on a static rope and bringing your legs up and down. Bring your legs up generates potential energy and letting them down releases that energy. Swinging just takes advantage of that and translates it into the forward and backward motion.
You are expending energy using your muscles to add to your momentum. If you think about how your legs move, they go behind the chains as you move back, then in front of the chains as you go forward. This changes your weight distribution to help even more.
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