Internal strength and willpower have a very interesting relationship.
One way of thinking of internal strength is as innate strength. When you use your innate strength to do something, it feels easy. It is when a task is beyond the realm of your innate strength that it feels hard. Think of how easy it is for a squirrel to crack a nut versus how hard it would be for you.
Innate strength for a human is not fixed but changes with use. Within limits, it increases in response to demand and atrophies in response to neglect.
Willpower is that which allows us to do that which feels hard, and therefore expand the capacity of our innate strength. While obviously very valuable to exercise, it is also finite and limited, and therefore must be conserved.
So to put it succinctly, if what you are doing feels hard, you are not using internal strength and instead are using willpower. This can increase your internal strength reservior as long as your willpower reservoir is full enough to draw on and the conditions are right. Conversely, however, the application of willpower is an indication that you are not using internal strength. To use internal strength you must de-emphasize willpower and stay within the realm of what feels easy. Internal strength, when properly applied, feels effortless to you and unstoppable to your counterpart.
