You don't know what barely off means? Or are you just being sarcastic? Barely off means exactly that, you will learn if you get good, instead of hitting right to your opponents forehand, you hit it "barely off" to his forehand. Following?
Instead of hitting to his forehand which is to his left, hit it more to his left, this throws a player off-guard, you can see Fed do it every match, he is famous for a slice to his opponents backhand, same shot again, then again, then rip it short, barely off what he did before, that's enough to take a guy out of his rythmm.
Yeah, so where is the footage of this error? Sounds like a really specific situation that wouldn't have a huge impact on a match.