As far as I can tell, and I'm no card-expert at all, what he was doing was simply "counting cards". It's a well-known card-sharping method, and forbidden in all casinos. They twigged to him as well.
I just wish to point that it is not 'forbidden', and cannot be, as long as you are only using your own brain. Of course, the casino does not like to loose, and if you win too much, they'll kick you out. I'm also fairly confident that the multi-player team card counting (that for instance the MIT student team applied -- multiple players play on different tables and count, and signal a 'high stakes' player when the card count is in their favor) is not really forbidden per se, but, since casinos are private property, the casino will not allow them to play or even stay if they see what they are doing. Casinos have also changed the procedure to make card counting more difficult, such as shuffling more often, or introducing fresh decks.
The thing is, Blackjack, like most other casino games, attract more players if the players believe they can win. If the casinos meddle too much with the rules, they will attract less players. Fortunately for the casinos, card counting and perfect strategy playing Blackjack is pretty stressful on the players, and only a minority of players can pull it off. Team playing reduces the stress level, but increases the risk of exposure. Again, it's not 'forbidden' in a legal sense, but it kinda sucks if you get kicked out, and other casinos get informed to not let you play anymore.
Another point: A certain amount of high stakes playing and winning is wanted by the casinos. It advertises the game, and attracts the more plentiful, but naive and untrained players to loose money.