The effect of moving cards in an envelope that the audience never guessed correctly.
Redeem the cards in the envelope right in the spectator's hand. Imagine you take out three cards, put them in an envelope, and place them on the table.
You then move the envelopes and ask your spectator where the cards are... The audience won't be able to guess the exact position even though they've seen it with their own eyes. And, with perfectly clean hands, you can swap the positions of the cards inside the envelope right in the spectator's hand.
Moreover, there is no need for a process, you can create your own moves and change cards. And do it until you want to stop.