As long as we are falling backwards, we will not be utilizing our innate uprighting system. This innate system depends upon our weight falling straight down. As infants, we had no trouble sitting. We had not yet developed a sitting-back habit.
Thus, as infants, as we fall straight down, tip forward and in, we create added pressure a little bit forward on the sit bones (with a small amount of our forward tipping weight going into the legs in front of our). These are optimal leverage points from which we bounce ourselves back and up, trampoline-like.
Stopping our forward-tipping at these ground-contact points activates our powerful, deep lateral hip rotators, both sides working together, which lift the pelvis back and up toward the vertical. This enables the deep, intrinsic muscles of the spine to lift the spine easily, with minimal effort. Once maximum height is reached, we fall and tip forward again, renewing the cycle.