I had thought there was a relation between purification and training in this manner, in pushing the body beyond bad habit, using exhaustion to create more efficient movement etc.
Purification ... ha ha. Yeah, right.
Been there done that - Aikido and elsewhere. What I noticed was that, when tired, you have to push yourself to continue to do it correctly. It does not happen naturally. When I observe, I see many people cutting corners when tired, and what you learn when tired and exhausted (if not doing it correctly) will sink in, and will be very difficult to undo later. In some people, exhaustion can lead to efficient movement - but only if the technique is done properly. And then ... what with different schools teaching things different ways, how do you know that what is being done is the best way to do it? Well, get tired, and concentrate on doing it well. If you are too tired, you MUST stop because the potential for learning 'bad habits' increases exponentially. I have seen that.