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If the holder uses Sandstorm, then it will last for eight turns, instead of the usual five. This also works even after the holder switches out.
Smooth Rock is generally overlooked as an held item due to the fact Hippopotas, Hippowdon, and Tyranitar have the Sand Stream ability, which summons permanent sandstorm. Even then, most of the Pokemon who benefit from sandstorm will want more than 8 turns to use it, and can't afford losing a moveslot to use the move Sandstorm, making Smooth Rock a poor item choice.