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A one-time use item that forces the holder to switch out immediately when hit by a damaging attack, even if they have not moved yet in that turn. This item bypasses trapping moves, such as Block or Wrap, Ingrain, and Suction Cups. The holder's controller is allowed to select the Pokemon sent out next. If a Pokemon with Sheer Force attacks a Pokemon holding Eject Button with a move that gains a boost from Sheer Force, then the effect of Eject Button is negated.
Eject Button could theoretically be used for Pokemon that set up temporary field conditions like Trick Room or Rain Dance, eliminating the turn needed to switch and allowing your sweeper to get in for free. However, in practice it is almost always a poor choice, as it is only single-use, and there is no guarantee that the strategy will work. In addition, the Weather Rocks and Light Clay are better choices for the temporary field conditions as they increase duration, thus more than making up for the switching turn, while Trick Room and Tailwind are pretty poor field effects in single battles anyway. In addition, Trick Room has -7 priority, so there is pretty much zero chance that you will be able to set up the field condition before being forced out.
The Eject Button is sometimes attached to Hitmontop in VGC doubles to allow it to "reuse" Intimidate and Fake Out, as well as potentially allowing a safe switch in. Other than that, though, the item is rarely seen as it is rather situational, especially in longer matches.
Battle Subway, 32 BP.