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The wielder's Attack and Speed are reduced by 50% for 5 turns after it switches in. If the user switches out, the counter resets. This ability can be removed by Skill Swap, Gastro Acid, Mummy, or Worry Seed. Slow Start will immediately activate whenever the ability is acquired, such as through Skill Swap or Role Play.
This ability makes Regigigas the worst legendary Pokemon that exists by a long way. This is probably the only ability comparable to Truant in how awful it is, so it really has zero competitive value.
Found in Dream World