However, when talking Koshi, (that would mean with weight downwards, heavy) then the point of movement, of rotation, of power, becomes the koshi which physically is the base of the spine, back of the hips. It actually becomes a relaxed point and an opening. The movements then become more like 'skating' smooth, no up and down. Moving from koshi and moving from centre, neither use upper body.
I can give you a reference for the explanation of koshi physiologically if you wish.
Aikido folks eventually get the sliding movement as a "sinking" over time. But in the beginning, folks tend to push from the rear foot. Any pushing from the rear foot to make a step raises the center a little bit. Think of a rear wheel drive auto. The front wants to rise when it accelerates.
Now think of a front wheel drive auto. The mass sinks rather than raises.