You spread out a deck of cards face up, and ask a spectator to choose any one at all. When she has done so, ask her if she wants to change her mind. When she has finally settled on a card ( for example the nine of spades ) you announce that you absolutely knew in advance which card she would take. In fact you even told a friend.

You ask the spectator to pick up the phone and call your friend, Phil Taylor, and ask him what card she chose. When she does so, your friend Phil answers: Why you took the nine of spades, didn't you?

the phone number, supposedly. But really you are looking for the key name for the nine of spades. (Actually, if you study the chart, you can have it emorized in a few minutes and then you never need to look anything up!)
The first name corresponds to the value; therefore nine equals Phil. There are four last name, one for each suit. In this case, spades equals Taylor. So when you tell her to call your buddy Phil Taylor, you are really giving the code for the card she selected. She calls the number you give her, and asks for Phil Taylor. Your buddy looks up his chart ad instantly knows she took the nine of spades!

Obviously you cannot repeat the trick for the same crowd!