From the Ken Brooke Magic Range. Performer shows a regular-sized envelope and mentions that there are four court cards in the envelope.
Example: Four Kings. He announces to the spectator that one and ONLY one of the Kings is FACE UP. The spectator gets a FREE choice of anyone of the four Kings-Clubs, Spades, Hearts or Diamonds.
When the spectator announces his choice the performer opens the envelope and takes out the four cards and spreads them and "lo and behold" the ONLY face up card matches the
one called out by the spectator. And it and ONLY it is a different colored back card than the remaining three cards. 100% accurate all the time.
If you are familiar with B'Wave (not currently available), this was it's precursor. Carry this in your pocket and you will always be prepared to WOW