Art takes 5 cards (at random) out of a regular 52card deck, and gives them to Ben.
The cards happen to be the 2d 4c 5h 5d Ks. Ben gives back the 5d to Art.
Ben then gives his partner Cam (under the door; Cam is in another room!)
the 4 other cards, in this order: 2d 5h 4c Ks.
Cam looks at the cards and tells Art: you got the 5 of diamonds!
Naturally, Ben sent Cam a signal; what is the signal?
NOTE: there is no "tricks" like upside down cards; the cards "do the talking".
And this will work for any 5 cards drawn by Art.