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Deals damage and resets all stat changes to 0. This effect will not work if the target is unaffected by the attack or if the target is behind a Substitute.
This move can be very useful at stopping Baton Pass chains, since it cannot be prevented with Taunt, unlike Haze. It can't, however, go through Substitutes, which should be taken into consideration. Another thing is that it doesn't affect Steel-types. This isn't a big deal for the most part, but it stops you from getting rid of Scizor's Swords Dance boosts. Clear Smog also helps against Conkeldurr and Reuniclus that try to boost their stats.
Haze is a non-damaging Ice-type equivalent.