Notice that in hitting we do not push or push in most cases unless we aim to stop the person. In hitting we move our limbs or anything else into the area of the hit and than it goes deep in. Think about it and practice. Your own expeirence is worth much more than any learned by reading when you need it.
We stand upright to spend the least amount of energy to keep our posture and navigate with ease and also to keep our senses (eyes, ears etc) in the best way to receive new information. When you are pushing or pulling or resisting those forces you align yourself along a greater force than gravity and that is of your muscles (for a while at least). Consider a wall or a post and try to push it over. You will notice your body aligning itself along the direction of the force you want to manifest into movement and the same goes for pulling. Work either alone using a wall and a door frame or a tree in the woods on pulling and pushing and note for yourself how you align yourself and at which point you start to lose the benefit of your senses compared to the aligned position. With a partner you can have push pull drills as you resist or not and work after you feel how you move when you resist to blend with the push pull and escape through this. For a more realistic view of things you can add a trunk of a car as the position to bring the partner to.