A sexual technique where, in the middle of foreplay (specifically when using your hand on their genitals) you suddenly announce that you are not using your dominant hand and adjust positions, increasing your skill and their enjoyment. In the movie The Princess Bride, Inigo Montoya was the greatest swordsman in the world. He was so skilled that he routinely dueled other swordsman with his left hand so it was more challenging.
Inigo Montoya: You are wonderful.
Man in Black: Thank you; I've worked hard to become so.
Inigo Montoya: I admit it, you are better than I am.
Man in Black: Then why are you smiling?
Inigo Montoya: Because I know something you don't know.
Man in Black: And what is that?
Inigo Montoya: I... am not left-handed.
Moves his sword to his right hand and gains an advantage
Man in Black: You are amazing.
Inigo Montoya: I ought to be, after 20 years.
Man in Black: Oh, there's something I ought to tell you.
Inigo Montoya: Tell me.
Man in Black: I'm not left-handed either.
Moves his sword to his right hand and regains his advantage