Cards Magic

You are given a regular deck of 52 cards. A magician enters the room and tells you to choose 5 cards as you wish and lay them on the table face up. Then he takes one of the five cards and flips it and re-sorts the remaining four cards in a line. He leaves the room and another magician enters the room at the same time. The second magician looks at the 4 cards, thinks for a few seconds and then tells you what is the 5th card.

How did they do it??