Quote from RS Wikia:
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"The mask ... has ten charges that are invoked at random against any combat opponents, not just slayer task monsters. A charge lowers the opponent's Defence level by 3-10."
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There you are. The chance that the charges are used is, as Rogiee said, completely random. The first time you attack a creature, the machine decides if a charge will be used or not. After all 10 charges have been used, there is currently no way to recharge the mask. The only way to get more charges would be to buy a new mask and/or kill cave horrors until one is dropped.
~ � Al�z
� ~