What I find important is hip control that comes from or is generated from the legs. If you look at the wazas I mentioned the hips are in sync, they parallel each other. Once the hips are in the desired position the leg(s) are the energy to move (not position) the hips on a desired line. Power is generated from the foot to the leg and into the hip. But the pelvises must be mirrored and a connection created (the defensive hip moves the offensive hip moving as one unit). If you are going to move someone with the great result, it is the pelvis you want to move. Where the pelvis goes so does the body. This is a very fundamental and well known concept in Aikido. Though, Aikido applies it differently under different circumstances, and sometimes isn't readily seen as being the same idea in Sumo.
The legs are just not limited to working in the only way I said they did. FWIW.