He said that counting cards is essentially illegal, but it's really hard for the casino to catch you. If you sit there and count cards and find ways to beat blackjack, good for you. But if you have a friend who is standing behind the dealer, using technology to count the cards and calculate what you should do and then that signal is relayed to you via a buzzer of some sorts, you have crossed a line and the casino is going to promptly escort you and your friend out of the casino.