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When Wonder Room is activated, the base Defense and Special Defense stats of all Pokemon on the field are swapped. Using Wonder Room while the effects are already active will cause the effects to cease. Wonder Room has a priority of -7. If a Pokemon with Prankster uses Wonder Room, it has a priority of -6. Any stat changes to Defense or Special Defense remain on the stat that was originally boosted.
Wonder Room has one of the most interesting effects in the game, but it should never be used competitively. However, there isn't much use for Wonder Room as it is far too gimmicky and there are better options to use. Reuniclus, for example, can use it to swap a Blissey's high Special Defense with its horrid Defense stat and then proceed to defeat it with Psychic. However, with Psyshock already available to it, there's no reason to waste a moveslot to accommodate Wonder Room.