A dice holder is a simple cheating device used by a dice mechanic to keep a pair of dice concealed on his person, until it is time to switch them into play.
The dice holder is pinned to the lining of the underside of a jacket. A pair of gaffed dice is securely held inside the holder and ready to be dropped into the hand in the blink of an eye. At the right moment the mechanic will drop his hand to the side of his body (a natural gesture, if done right at the right moment) and curl the tips of his finger under the lower edge of his jacket. The dice holder is strategically placed at the exact position where the hand would naturally fall, if a person were to relax. Once the dice are in the hand, the hand knows what to do.
The idea with most scams that use gaffed dice is to switch the dice during a roll. We are not going to discuss dice switching techniques here, but I do want to mention about the clean-up (i.e. getting rid of the outgoing dice) because some clean-up techniques utilize a gaff that is hard to photograph. Also, it is a gaff that I do not wish to expose in detail, so I will just offer a short description.
The gaff is commonly called a web and it is basically a large pocket added to the inside of the lining of the jacket. To switch the dice the mechanic will perform a motion that brings his hand close to the front edge of his jacket and toss the dice inside the web, at the exact moment when the gap between his hand and the jacket is closed. The exact moment when this clean-up is done will depend on what exact dice switch the mechanic is using. In some cases the clean-up will happen when both pairs of dice are in the hand (in other words, during the actual switch), and in some cases it will happen after the switch.
A web is a utility device that is used for various reasons. Professional shoplifters also use this gaff for dumping small objects, such as switching expensive jewelry for fakes in the blink of an eye. However, in the professional shoplifter's jargon this gaff is commonly called a booster bag.