Actually, you are more likely to get comped than detained if they detect you counting.
Very few people both actually understand it *and* are capable of doing it well enough. So the response is much the same as to anyone else with a system: "welcome."
Casinos *do* have significant power to detain for cheating, but the 21st centurry is a bit different than the 1960s . . . (no more back room beatings, cornfields [which we never had, anyway!], . . ._
Cheating, though, is another story. If the "counting" goes in any way more than your own memory, it's probably cheating, and (at least) a gross misdemeanor.
I humbly disagree with Hawkins. Casino security is specifically empowered by Nevada law to detain you for SUSPICION of a felony. Cheating at a casino game is a B felony in Nevada. While counting cards does not fall within the definition of cheating, they could detail you for suspicion of cheating. Casino security has a long history of "roughing up" card counters and cheaters. Basically, they would likely detail you on suspicion of cheating, possibly rough you up, and then let you go. At that point, the evidence of any incident magically disappears, and you are left with unexplained injuries and very little evidence to make a civil case against the casino.
If casino security suspects you of a felony they are supposed to detain and immediately call metro. They can question you, but you always have the right not to talk to them.
If you are simply at a single-deck blackjack game, estimating the outcome by the number of face cards that have been dealt, you can probably get away with it. But movies like "21" don't happen. Casinos will not let you get away with it. Once you start winning substantially, they will put a stop to it.
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