Regarding moving forward with the ankles in the same position, think about the biomechanics. If the ankle does not dorsiflex or plantar flex while the leg is extending, what happens to the center of motion? It moves forward. So, extension of the knee and hip is a movement pattern that moves the CoM forward relative to the base of support. Similarly, flexion of the knee and hip moves the CoM aft relative to the BoS.
Adding in ankle flexion you have the primary mechanics to manage the relationship of CoM and BoS.
Adding in ankle flexion you have the primary mechanics to manage the relationship of CoM and BoS.