[DifE-Q]

So can you explain to us how one builds character then - how else is it (character or self-discipline) built but by resisting against a natural pull, thought, or reaction?

The Spartans to General Patton used similar techniques in building the character of their troops and their fruits cannot be disputed.

The author is not claiming that this only will build character, but this is a small tool to to work on building character yet another way. Alone perhaps it would be meaningless, but taken together with his other approaches it can lead to significant results. It may be only a tree in the forest and it takes trees to build a forest; this method can be one of those trees.

Or if you like, to return to the analogy of resisting something in order to become stronger: This is just another exercise, like the plank (among many), that used alone may not produce much; but taken together with a whole exercise program WILL make ones whole body stronger.