Fixed Point and Lever
This following diagram explains what is demonstrated in the video:
The rear foot and the fixed point form a line, which aims at the centre (spine in most cases) of the opponent. The hand/elbow and the same fixed point through the opponent's structure form the lever arm. By bringing the lever towards the line, energy shoots towards the opponent. Note that there are two applied forces, one is used to push towards the fixed point, and the other is pushed towards the line. The two forces are created by two different parts of the body. While the lever is squeezed towards the line, the fixed point (and the line) cannot be moved (tossed).
In the video, different fixed points are selected as well, e.g. rear kua, middle of the trunk, front kua.