It is done with a single gimmicked card. That's how blaine did it, and that's how everyone I have ever met that is good with cards does it.
You can also do it a way without a gimmicked card, but it isn't anywhere near 100%. It's probably the way most of you first thought it was done, I know it's the first thing I thought.
I don't want to turn this into a debate, but no, Blaine did it with a normal deck of cards, doing it with a gimmicked card is redundant. I've never ever seen anyone do this with a gimmicked card unless they are using a forcing deck, that is.
Watch it several times. It is a mental force. I think it was called the Optical Riffle Force. It's a fact that when you riffle a deck of cards, you slow down right at the very end.
When I perform this mental force, I have never had anyone tell me that they saw a different card. So, to me, it is 100% accurate - technically, it isn't, but when i perform it, and I've used a mental force thousands of times - it has never failed. Not to sound arrogant or anything.