Re: Jamming a technique
As a rule of thumb, I tend to go through a technique once or twice with nage before I actively work to exploit holes. I also tend to not exploit holes with people who have less experience than I -- at least not at first.
Often, I say something before trying to jam a technique or reverse it. I'll say, "Hey, I feel something odd, let's do it again."
With some people, though, I simply try the reverse or atemi, or whatever. It's a great way to learn how to be a better attacker. It often happens that a hole I thought I could exploit turns out to either (a) leave me terribly open or (b) it's really not there.