Although we commonly think that we balance on our feet, we in fact balance upon the talus, the top foot bone. All of our weight sits directly atop the talus which, together with the tibia (the larger of our two lower-leg bones) forms the ankle joint. The talus is a tiny perch upon which to balance; only a few square inches in area.
To optimize our standing, we need the force of our falling to fall straight down through the talus. Doing this tips us forward and in a tiny bit, taking us towards the strong part of the foot, the longitudinal arch. This spreads the foot bones, which activates the plantar muscles that jump-starting our innate uprighting/extension system.