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If the holder uses Rain Dance, then it will last for eight turns, instead of the usual five. This also works even after the holder switches out.
Damp Rock is a staple on Rain Dance teams, commonly seen on their leads and often on the back-up Rain Dance users. It provides extra turns for the other teammates to set up and sweep, as five turns are often too short to accomplish much.
Damp Rock is one of the most useful tools available to Rain Dance teams, extending the duration of their rampage. Five turns (working out at three, counting the turn on which Rain Dance is used, and then the switch to the appropriate sweeper) aren't quite long enough, and switching becomes more frequent in order to set up Rain Dance again. It provides three extra turns before Rain Dance must be used again.
On bulky leads such as Bronzong, Leftovers recovery will be sorely missed. On offensive Pokemon such as Ludicolo, the loss of Life Orb can be quite disappointing. On frail leads, the choice between Focus Sash and Damp Rock can be difficult. Without Focus Sash, the lead may not survive faster / priority attacks to even set up Rain Dance, but with it, the turns will be almost worthless.
There are several ways to utilize the turns Damp Rock provides to maximize efficiency. One is Exploding or U-turning to dent the opponent and allow the Rain Dance users to come in unscathed. The second is placing it on a Pokemon which can take advantage of the rain itself, although this takes up both an item and a moveslot. Damp Rock can work on many types of leads, but be aware that frailer leads in particular may not survive without a Focus Sash.