You choose which pile to put into your hand and which to put into your graveyard. You can’t look at the face-down pile until after it’s moving to its new zone.

A pile can have no cards in it. In this case, you’ll choose whether to put all the cards into your hand or into your graveyard.

In a multiplayer game, the targeted opponent may choose what information to share with others if that player wants advice on how to split the cards, but that player doesn’t have to share any information at all.