This should be it -- really.
The shape of the drop is not linear. The weight usually drops in a spiral.
If the weight drops in a vertical line (straight down), it will bounce back in another vertical line (straight up). If it drops in a spiral, it will bounce back in a spiral, which is easier to redirect.
The drop is made using the koshi, which has a rotary motion (not simple rotation in a horizontal plane). The shape of the koshi motion translates into a spiral drop. You can sometimes see this in the movement of the knees.
I hope that this has helped to make the subjects of weight placement and pivoting a little clearer. I should add that I would not teach this to beginners or even intermediate students. I would wait until the student was fairly advanced. In my opinion, it is best for beginners to develop clean and strong linear basics first.
Respectfully,
Charles C. Goodin
