Holdout (gambling)

[1] Some holdout devices are extremely simple and require moderate or advanced manipulative skill to be used properly.

Even if those machines are complex mechanical apparatuses, they still require a good level of skill from the cheat's part, to be used so that they work but are not noticed.

[1] The Kepplinger holdout device and many others were described in detail by the magician John Nevil Maskelyne.

[2] The main purpose of any holdout device is to temporarily hold a card out of the game, so that the cheat may retrieve it at some later convenient time.

In effect, this is as if the cheat was dealt a bonus card, so that he may decide which combination of cards he likes best and finally discard the unwanted one, only to possibly use it on the next round (or at least switch it for a better one).