I thought this was a very interesting article.
I take it to mean that effort given is a trainable quality in itself that must be sensibly managed like any other qualities. It is apparent here again the effect of stresses outside training, and also the importance of structuring the training environment to build good habits.
Too much will power demanded (more demanded than the athlete is able to deliver) and an athlete will simply give up, or slack off. But if attention is not paid to increasing willpower (‘exercising the willpower muscle’ is the metaphor of the article), than you are missing out.