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Deals damage and has a 100% chance to lower the target's Speed by one stage.
Icy Wind's Base Power and accuracy are rather poor by competitive standards, but its secondary effect makes the move somewhat useful. Some Pokemon that are weak to revenge killers can use this move to catch a switch in, reducing its Speed by one stage and thus averting the danger. However, this strategy is not commonly seen since it is unreliable.
However, it's a different story completely for Icy Wind in doubles and triples. Being able to slow down both or all of the opponent's Pokemon in one turn can easily open up a sweep for a teammate, not to mention that some of the most threatening Pokemon in the game are hit super effectively, such as Landorus, Zapdos, and Garchomp. A common user of the move is Cresselia, which, thanks to its gigantic bulk, can easily cripple the opponent multiple times with Icy Wind, and will have multiple tries to use the move even if you mispredict the opponent's Protect or if it misses.
Ice Beam is a stronger option that has a chance to freeze the target.
Glaciate is a more powerful equivalent.
Electroweb is an Electric-type equivalent.