Each stack matches exactly.
The magician gives the spectator one pile, and places their cards face down on a grid on the table.
The spectator places their cards face up on the magician's cards trying to make a "match"
The magician collects each pair and places them into one pile.
Amazingly as the magician deals them back out, EVERY SINGLE CARD matches EXACTLY.
All pairs match, even though the spectator had free choice.