Remember Sir Isaac Newton part one

Newton's Third Law of Action & Reaction states that for every action - force - in nature there is an equal and opposite reaction. Think of that in your bow hand and your left hand. A good reason to not pound the left hand fingers is the amount of tension it causes the hand in counterbalancing that amount of pressure by the left thumb. A good reason not to press the bow too hard is the counterbalancing pressure it puts on the right thumb. The law applies to every action. No exception.