Interestingly I've found this same approach from good coaches across completely different niche sports. I imagine this phenomenon has been discovered a number of times by various smart people. It certainly wasn't intuitive to me, but since learning to use affirmative advice in real-time sport situations, my advice got noticeably more effective.
My karate sensei has a terrible habit of showing how not to do things more than he shows how to do them, drives me crazy.
Eg: If you're doing a sport where leaning forward is bad, avoid telling yourself 'dont lean forward' as your mind only hears 'lean forward', therefore reinforcing the thing you're trying to avoid. Alternatively, tell yourself 'lean back' or 'stay straight' or whatever you're focusing on for that maneuver or drill.