Don't believe me? Again, you'll need to stand up. Try pushing
down with your right foot, but not moving your pelvis. Can't do it,
can you? Either you move to the left because the leg pushing down
throws off your static balance, OR you move to the right because your body
anticipates the need to apply pressure to the right leg, and unconsciously
shifts your weight. Either way, however, you cannot push down on
your right leg without shifting your pelvis.