The magician puts a deck of cards in a closed box on the table. A spectator thinks freely of a card and say which one it is. The magician takes the deck and spread the cards in a line between his hands and shows how one card is is facing down. Clearly, this is the card chosen by the spectator.
It has a huge impact on the public as the card is freely chosen. The viewer can choose any deck card.

Being a gaffed and manipulated deck is obviously not sealed.