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The user and the target have their Defense and Special Defense stats set to be equal to the average of the user and the target's Defense and Special Defense stats, respectively, rounded down. Stat stage changes are unaffected. The effect on each lasts until they faint or switch out.
The fact that Guard Split ignores the stats changes of both the user and the target makes it an inferior Guard Swap.
Theoretically, it's supposed to be used by a frail Pokemon on a bulky one. Averaging the Defense and Special Defense would mean boosting the user's own defenses while lowering the opponent's once bulky stats, making an impassable wall easier to kill. However, most users already sport great defenses regardless (the exceptions are Smeargle and Alakazam), so using this move would be entirely counterproductive.
Power Split works identically; however, it averages the Attack and Special Attack stats.