Originally Posted by Stinky
I guess the way to explain Choice items is like a piece of clay a Pokemon holds. If they use a move while holding this piece of clay, it makes an imprint on the clay and they have to keep using it. When its given to another Pokemon, it's "reset" and has no imprint on it. When the new Pokemon uses its move, it imprints the clay again. The clay does not know that it used a different move the last turn
Or we just go by how Smogon says it. If a slower mon is Trick'd a Choice Item, it's stuck with the move it uses. Simples.