This is a popular effect performed around the world by leading magicians.
Ask the spectator to choose a card at random and sign it. They then place the card in their mouth. You do the same and somehow the cards swtich place, their card is now in your mouth and your’s in their mouth.
An intimate trick, the DVD has 30 minutes of teaching plus multiple live performances with voice over from Wayne Houchin highlighting key moments in the presentation.