As was said, they catch you by watching how you vary your bets, which is why the MIT system was relatively successful. There are various methods to it, all with various pros and cons (accuracy, difficulty, etc.). Also, counting cards is NOT illegal in the US. You may be kicked out, but the days of dragging people into the back are gone. In fact, in Atlantic City, you can't be kicked out even if they know you are counting cards. They can make it difficult for you by shuffling after every hand, but they can't even kick you out. As far as the smartphone app, I have used one to practice, but I've never seen anyone try to use one, and it is illegal/considered cheating to use a device. Counting to yourself is a form of strategy. If you think about it, if there are a lot more low cards than high cards at the start of a shoe, then you know there have to be more high cards towards the end, so it is more likely that you will get a blackjack. Counting cards simply involves paying more attention to this.