Dave de Vos
I was afraid that this would be the general answer to my questions, but I am still hoping that someone can offer some information.
1) It is a way of using your body to create a lot of force with physical efficiency, a way to increase your range of motion while staying balanced, and a way to take an opponent's balance without force and clashing.
2) Yes, the videos point to 'internal strength'. But whatever they do it or not depends on how they feel in real life. I would not judge until I have crossed-hands with the person.
3) Yes you have to do a lot of solo exercises. You are basically re-wiring your body and undoing movement habits that prevent you from expressing bodyskill. The exercises target and condition parts of the body that allow you to move with whole body. For example, if your abdomen moves, then your arm and legs, akin gear-like dynamic, should move as well.