All these are traditional rules. If you break these rules, then the power will not be as good. The variation is controlled by the waist. But if you let your left chest withdraw backward a little bit, and turn your wist to the left a little bit, then the power is there again. So this variation is on the chest, and controlled by the waist, and finally shows on you fingers, If the whole body is relaxed and sensitive, then strong power will show on the fingers.
People are animals, but with senses.
It is the natural reaction of people, but materials are different. An example of this is a hanging sandbag. It is motionless hanging there. If you punch it, it will move forward and backward. The way it moves is on a fixed route.