I agree with bsms in the above.
It's the balance is over the feet. As one folds over the jump, the shoulders come down (of course in front of the feet) and the butt goes back to compensate. Otherwise, one is "ahead" of the horse or one's center of gravity is in front of the horse's.
Conversely, the infamous "chair seat" would be when the rider's center of gravity is behind his/her feet. Think about the alignment when one sits in a chair. Where is the "sitter's" center of gravity?
In riding the rider's center of gravity should be directly over one's feet. Or as my instructor says, if horse were pulled out from under the rider, the rider would land balanced on his/her feet.
For jumping (as my instructor also says), the crouch a baseball player or tennis player assumes "at the ready" is really the fold for jumping. It is the most balanced position for a person on two feet. The center of gravity is as low as it goes.
ETA: While the "vertical pole" through the shoulders, hips and ankles works when the rider is fairly straight, it changes as the rider crouches into a two point. I'm always told the same angle that occurs with the leg should be repeated at the waist. So the pole really will no longer pass through the hip and shoulders as those two weights go in different directions as the body is compacted.