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If the holder uses Hail, then it will last for eight turns, instead of the usual five. This also works even after the holder switches out.
Icy Rock may seem a very useful item like its rain counterpart, the Damp Rock. However, Icy Rock doesn't see a lot of usage in competitive play, mostly due to the fact Snover and Abomasnow have the Snow Warning ability, and are playable in all tiers. Even then, unlike the Pokemon who benefit from Rain Dance, those who benefit from hail generally appreciate more than 8 turns of weather, since they are more defensively and stall oriented.