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If the holder has 25% HP or less and uses a move, then it will consume the Custap Berry, and the move used will move before all other moves in the same priority bracket for that turn only. For example, if the holder were to use Spikes and the opponent to use Taunt, then if the Berry were to activate, Spikes would go before Taunt irrespective of the combatants' Speed stats. However, if the opponent were to use Bullet Punch or another elevated priority attack, then Bullet Punch would go first. Similarly, a Pokemon with Prankster will move before a Pokemon using Custap Berry when using a non-attacking move. Using Pursuit against a Pokemon that is switching out does not activate Custap Berry. Natural Gift becomes an 80 Base Power Ghost-type attack when the user holds this Berry, but the Berry is consumed after use.
Theoretically, Custap Berry has many uses indeed. Sturdy Skarmory or Forretress can be sent out first, survive any hit thanks to Sturdy while setting up Stealth Rock, and then consume the Custap Berry in order to jump ahead of the opponent and lay down an extra layer of Spikes. And who can forget the dreaded Custap Wobbuffet, who could survive a hit thanks to its huge defensive stats and get in an extra move before it's KOed—supposedly the very reason for the use of Unnerve Mewtwo in Dream World Ubers?
Can be obtained by competing in the 2012 VGCS Winter Battle, the Global Battle Union Wi-Fi tournament, or found on the Share Shelf of a player who has competed in the past.