None. It'll be like a compulsion to follow it. They could try resisting, I guess. Like, if you ordered someone to chop their hand off, they might try reeeeally hard not to, because, who the hell would want that?

But they wouldn't be able to ignore the urge. So, you'll either have someone with one less hand, or someone who won't be able to focus on anything except the fact that they want to cut it off.