When the heel first touches down, we have to deal with lateral momentum. Lateral momentum exists because we make initial heel contact having come from the big toe of the opposite foot. In other words, we approach heel contact on a diagonal. By successfully committing our weight straight down as we move up to the talus, the heel rocks inwardly at the pivot point -- simply because the center of the talus is medial to the pivot point. This directs us right into the longitudinal arch, the strong part of the foot -- and leads us toward the spot on the ground where the heel of the other foot will be touching down in a moment.