A couple yudansha where I train are very good at non-verbal correction. I do the technique, then they do the technique slowly with great emphasis and control on one part. Then I try, and the next time either some smaller nuance gets emphasized or whatever.
Very cool - it's very much a conversation, and it's using the part of my brain that actually learns the technique. As a very verbal person, I get really caught up in my verbal mind when I get verbal correction - this is bad, it puts me away from my body.
So, since it's bad for me to talk and train, I try not to correct anyone unless they ask or seem really frustrated.