When people talk about a card mechanic, they are talking about a form of cheating. There are a bunch of ways that someone can go about cheating at cards from colluding with other players, sleight of hand movements like stacking the deck on the bottom, or the use of something physical like marked cards, cold decks, or holdout devices. A card mechanic, which is also known as a card sharp, is a cheat who has a specialty in the sleight of hand movements used to manipulate the deck. Magicians use this kind of thing in order to keep track of either one card or sometimes even the complete deck, and card mechanics want to know the same information so they improve their chances of taking risk out of the game.
There are a lot of different forms of cheating techniques using simple sleight-of-hand movements, but the easiest one to accomplish is something called dealing seconds. With just a little practice someone could put this to use in a not-so-friendly home game because all that is required is that the second card from the top or bottom is dealt instead of the customary top one. This move has also been called “second dealing” or “bottom dealing.” If there are untrained eyes watching the action then even a novice dealer can learn with a little practice to deal the second, the bottom, or even any card out of the deck without being noticed. “Culling” can also be used in which the dealer with a little better dexterity could find the card that he or his partner needs, place it somewhere in the deck where he can find it, and then dole out that card to the player that needs it.
The easiest way to identify this type of cheating is to see how the deck is being held. The “Mechanic’s Grip” is the form used by cheaters. A right-handed dealer will hold the deck in his left hand, three fingers on the edge of the long side of the deck and the index finger on the outer right corner. This makes it easier to deal from anywhere in the deck, but it’s extremely easy to take the second to top or bottom card.
Be careful though because a mechanic’s grip alone is not enough proof to accuse someone of trying to cheat you. You’ll need a little more proof than that, but if you suspect cheating is going on, then it’s best you find yourself another game anyway.