First of all, why would you want him to teach you a sleight that he apparently couldn't even pull off properly himself?
-Dave-
Did you see him excuting the move to judge him?
And, anyone experienced at cheating at the card table knows that accidents happen, and the result is very, very bad. Someone can miss up a second deal, and have his hand cut off ( see Ortiz's book on Gambling for more info ).
Even when doing normal sleight of hand, people miss up and it happens to everyone of us even if you practice 10 hours. ( pass, double lift .. etc ). If you say that you never missed up a move when you perform, then ...
Point is, I would never judge him doing a sleight without seeing him doing it.