you actually "tap" the dice and they magically become fair, or unfair. from what i can tell, some of them contain a fluid...
Those dice are called ‘Tappers’. The interiors contain oil and a drop of mercury in a dumbell shaped channel. When the mercury is in the center cavity, the dice are fair. When the shooter wants the best of it, he taps the surface ‘for luck’. This jars the mercury to the corner of the die and allows the shooter to throw a set of loads.
This and much more is described in “Scarne on Dice”, which is a 500 page textbook on everything dice. He includes a large section on avoiding cheats.
Scarne also describes in detail, a number of crooked dice shots, such as, the lock grip, the blanket roll, the backboard control shot, the whip shot, the Greek shot, the spin shot.
Many of these shots were developed by soldiers in WW1 an WW2, who filled suitcases with cash using them against unsuspecting fellow soldiers.
For the spin shot, the shooter gives the dice a fair shake, then glimpses at the numbers facing him when his palm is up. If he’s placing numbers and looking to avoid a seven, he makes sure the numbers on the opposite sides don’t add up to seven. If he’s looking at a five and a four, he knows the opposite numbers are two and three. Those will be the skyward numbers.
Then, palm facing down, he slides the dice across the surface, but gives them a spin so they circle each other, but never turn over. The three and two remain skyward for a five.
A shooter used this shot effectively at Foxwoods for a few rolls. The PB spotted it, and he was warned ‘do it again and you’re out of here in cuffs'.