I am surprised to see any mention of this trick.
Yes, I know Doc Daley's effect can be done with no gaffs, but actually showing them you have the other 2 cards seems to take the trick to the next level IMHO.
In fact, you can do a black jack demo showing 4 cards, a black ace, black jack, a black ten and red nine.
You tell the story about the 4 card casino that plays only one game, black jack using just 4 cards. As long as you do not get the 9, you are guaranteed to atleast tie.
You then patter how he gave you first choice and you got the ten. He took the middle two leaving you with the bottom card, the nine. He placed his cards back to back and showed me he had BLACK JACK.
He went to take my money, but I said look at your cards again. He saw he had the 19 so I had black jack. They will not let me back in the casino any more.