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If sandstorm is the prevailing weather condition, the wielder's Speed stat is doubled. If the sandstorm ends or a new weather condition is brought into play, the wielder's Speed returns to normal. In addition, if a non-Ground-, Rock-, or Steel-type Pokemon has this ability, it will not take damage from sandstorm.
This ability is part of what makes Excadrill the huge threat it is. Incredible Attack isn't vicious if Excadrill can't get a hit in, but with decent Speed already which is then doubled by Sand Rush, Excadrill exceeds even Kingdra in weather domination. Follow this up with Swords Dance and great coverage, and Excadrill becomes an unparalleled weather threat—indeed, this was the factor that got Excadrill eventually banned from OU. Sandslash and Stoutland are the only other notable wielders of this ability, but they doesn't even come close to Excadrill's pedigree.
Found in Dream World