As alluded to in another comment, this post kind of makes a lot more sense if you've read The Inner Game of Tennis.

It kind of glosses over competence of practice, but the TL;DR is once you've built up some competence with a skill, staying in a constant state of effortful tension won't give you better results. Entering flow state requires getting into "unconscious competence" effectively.

...which is effectively a reframe of how The Inner Game of Tennis says it: to practice your still with non-judgement while you do it.