It's not exactly true FP. When you play a sport there are many occasions where you need to brace with two legs in order to apply force to something, or it could be to get in a ready position because you need to make a decision to sprint/step/jump/dive this way or that way. Think of guarding somebody in basketball, trying to read a return in tennis, defending in football, goal tending etc. The athlete is balanced and ready to react/attack. Hitting a baseball is different than all the things I described, but different than sprinting to I guess. I just know having two feet on the ground in a ready position is a strong balanced position, much stronger than dynamically balanced with one foot on the ground and one in the air
I do not disagree with the bold, believe it or not. Not completely anyway, as there are exceptions . If you remember, I commented that swing launch can occur at toe touch, shortly prior to toe touch, or shortly after toe touch. Furthermore, the rearward launch is partially about acceleration, not max speed. As the acceleration increases, the pressure felt at the front foot will also increase. The feedback allows the acceleration to continue in order to reach max speed.