Puzzle: Naming the fifth card

I have a trick that I learned from my mother years ago. It involves two partners. One starts, while the other is absent, by laying out a group of cards, and having a volunteer pick one.  The partner arrives, and the first asks about each card in turn “is this the correct card?” and the partner stops him at the correct one. Sure fire. Silly. I’d have to show you a picture.

I have another one I learned about 10 years ago from my graduate advisor. It involves partners again. In this case, one asks a volunteer to draw 5 cards, and returns one card to the volunteer to conceal. He then holds up the remaining four cards. From a distance the second volunteer identifies the missing card.

I call it Matthews’ trick because one day I saw Matthews’ at a conference holding up four cards at a distance, and guessed what he was doing and shouted out the card – and it was the same trick, I was right. Now when we meet at conferences we pretend not to know each other, and play the trick.

Yes, we signal. But we signal by arranging the cards. How?

(I teased and then taught a bunch of freshmen on the trip to DC this past Thursday and Friday. They were good. And good sports.)