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Deals damage. Reversal's Base Power increases as the user's HP decreases, with a range of 20 to 200 Base Power. Base Power is calculated thus:
Alternately, you could do it as a direct function of your % HP, although this value could be off on borderline cases due to rounding.
Reversal is mostly famous for its use on cheesy Substitute + Salac Berry combos from past generations, which were easily foiled by residual damage. In a generation that lacks this combo due to the current unavailability of the pinch Berries, it is barely known to be on any Pokemon, but some occasionally find success in the strategy. Strangely enough, perhaps the most prominent and successful user of Reversal by far is Dugtrio. At full power, Reversal gives Dugtrio the power it needs to OHKO the likes of Blissey, Chansey, and Tyranitar, as well as deal serious damage to Ferrothorn and Politoed. Arena Trap works very well with this move, since there is little that the opponent can do about Dugtrio once it has initiated its strategy.
Flail is a Normal-type equivalent.