Rose severed Jack’s hand.
Allowing someone to use an axe to release you from handcuffs would be insane, especially when that person closes their eyes before firing the shot. And if you look closely at Rose’s axe swing, you’ll notice she wasn’t supposed to make that shot. If it weren’t for the movie magic, she would hit Jack just where his wrist is, thus removing his hand. That would also liberate him from his restraints, but in a far more painful manner.