Another form of houdini would be when a guy is fingering a girl while she's sitting on his lap. And then he takes his fingers out and in a sneaky way substitutes them for his dick and make it feel like he put his fingers back in. Once he gets his dick in, he put his hands in the air and yells "hey look! NO HANDS!"
Guy 1: hey man, i pulled a houdini on my girlfriend last night.
Guy 2: awesome!
Guy 1: yeah, she had no idea how my dick got in there that quick!