I had one instructor who never rode a bike during class unless he wanted to demonstrate something. He was a roadie, and he walked around to check on everyone. He'd look at people's form, make suggestions, whatever. He never touched anyone's bike. That is completely uncalled for. Luckily, I've never had an instructor who would come over and start messing with the resistance. It's a good thing I can quickly bring that flywheel to a halt without using the break, because I'd probably do so and then smack him/her. Asthma, (preexisting) injuries, or not, that is not ok.