Five cards are shown individually front and back.  Four cards are black with blue backs and one is red with a red back.  (Colors may be reversed in your set). Each card has a hole punched through it in its outer edge.  The cards are squared up so all the holes line up.  Magician then pulls the red card back so its hold is no longer lined up.  A small lock is now brought out and spectator locks it through all the blue cards.  Magician explains that the red card's edge is razor sharp and demonstrates this by pushing the red card up and through the shackle of the lock.  Each card is now shown individually to the locked on the lock.