[mself]

Here’s another version that helps clarify it...

There are three doors, one has a prize and the other two are empty. You pick a door (say, Door 1). Monty then says that his assistant will move whatever is behind Door 2 (if anything) to Door 3. He then opens Door 2 to show you it is empty. Would you switch to Door 3? Of course you would.

But Monty’s assistant is lazy, so instead of moving the contents from behind one of the closed doors to the other, he just asks Monty to open the one that he knows is already empty. The result is the same.