The magician fans out groups of cards for one spectator after another, asking them to remember their choice of any card they see but not say its identity out loud. After four or five spectators have chosen cards, the magician reads their minds and reveals which cards were chosen.
The spectators don't have as free a choice of cards as it appears. When the magician fans the cards, fingers are placed over the top of the fan to obscure the pips of all cards but one -- the one the magician wants the spectator to choose. The spectator really has only one card to choose from.