The performer wants to show that the hand is quicker than the eye.
He shows four 9 of Hearts. He explains, "First of all, I will do things very slowly. If you don't follow what I do, I will stop right now! Otherwise, I will go faster!"
The performer turns one card face down very slowly, and put it inside the face up pack of cards. These actions are clear to the audience.
"Ok, I will try to go faster now."
He does a very fast move. The audience feels that something happens, but they can't see anything! Even so, the performer shows that all the cards are now face down!
Now, he left the audience several chances to see what happens; the spectators don't see anything, but one more card turns upside down each time!
The last card is the last chance. "And if I go in slow motion, as with a DVD, picture after picture, maybe you can see what happens?"
The performer acts as if he pushes a "slow motion button" on his arm. From now on, the actions are made slow motion: in full view, he turns the last card face up with the tip of his fingers.
"No I'm joking, I don't do it that way. Would you like to know how i really do it? OK. I will put one card face down, and you will see how that card will turn upside down. I don't do anything! These cards turn upside down by themselves, like a sock! Let's imagine, you have a sock in front of you. You turn it inside out. What happens? The outside becomes the inside, and the inside becomes the outside, but progressively... Let me show you."
And now, you show that the 9 of Hearts turns "downside up" progressively, step by step!
You can pass everything for examination.