Preferential shuffling is not a defense tactic; it's a practice that's more akin to dealer cheating. The term is often confused with "shuffling up," which is a standard defense tactic, or casino "countermeasure."
The term "shuffle up" was coined by Ed Thorp in his book Beat the Dealer. It refers to a dealer shuffling excessively or in direct response to a suspected card counter raising his bets. Here's Thorp's classic description of a real incident:
"We immediately began to win, but within minutes the owner was on the scene. In a panic, he gave the dealer and the pit boss instructions.
"Then an amazing performance began. Whenever I changed my bet size, the dealer shuffled. Whenever I varied the number of hands I took (by this time I could play from one to eight hands at one time and faster than the best dealers could deal), the dealer shuffled. The dealer against whom I had played last in my practice session was standing in the background (had she "fingered" me?), saying over and over in reverent tones how much I had advanced in skill since the other night. Finally I happened to scratch my nose and the dealer shuffled! Incredulous, I asked her whether she would shuffle each time I scratched my nose. She said she would. A few more scratches convinced me she meant what she said. I asked whether any change in my behavior pattern, no matter how minute, would cause her to shuffle. She said it would."
Since a counter raises his bets when the count is high, shuffling at that point effectively erases the existing advantage. While it may not sound "fair," shuffling up is considered a legitimate countermeasure in the ongoing battle between card counters and casinos.
Preferential shuffling, on the other hand, is decidedly less innocent. This practice is implemented by dealers who know how to count cards. They count as they deal, shuffling the deck whenever the count goes positive and continuing to deal when it's negative. Hence, unsuspecting players are continually betting into negative-expectation situations, rather than getting the natural and random mix of positive and negative decks that unmonitored dealing produces.
Is it cheating? In his book Blackjack Attack, Don Schlesinger calls preferential shuffling "unquestionably, beyond a shadow of a doubt, blatant, arrant, outright cheating." Schlesinger continues:
"The laws of all mainstream casinos insist that, to ensure fairness, the cards be dealt in 'random' fashion. Without belaboring the semantics of the term, it is clear that deciding to continue to deal when the player is 'handicapped,' while refusing to deal when that same player is 'advantaged,' destroys the randomness of the dealing process. To claim otherwise is absurd."
We agree, but the silver lining is that this happens only in single-deck games, where the dealer (rather than a cut card) determines when a shuffle takes place. Not only are single-deck games increasingly rare these days, even where they're dealt, preferential shuffling isn't something you'll run into very often anymore.
