He discovered a defect in one of the decks print, it called edge sorting. Only certain casinos
used the deck.
"The cards are supposed
to cut off the pattern so that each edge of the card is identical to its opposite. However, these cards, manufactured by Gemaco, featured a defect that meant if you turned some cards around, and left the others the way they were originally, never shuffling them so that the edges were all mixed up, you could identify flipped cards from non-flipped. This is possible because in casinos, you're generally dealing with pre-shuffled decks, and if a shuffling machine is used, the vertical orientation of the cards will never change. And so, Ivey and his partner asked that the "good" cards for baccarat be flipped."
He would initially lose a couple hundred thousand then request the certain decks. Calling it superstition. Then because he had given so much to the casino and was going to play a million more he would ask for certain small changes and they would oblige to get his business. He would then play a strategy knowing the cards of when to pick which hand etc. Walked away with a couple mil from multiple casinos before they figured it out.