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Deals damage and traps the target for 4-5 turns, dealing 1/16 of the target's maximum HP at the end of each of those turns and preventing the target from switching. If the user is holding a Grip Claw, it traps the target for 5 turns all the time. If a trapped Pokemon uses Rapid Spin, it will be freed. If the user is holding a Binding Band, the damage dealt at the end of each turn increases to 1/8 of the target's maximum HP. The target can still switch out if it is holding Shed Shell or uses Baton Pass, U-turn, or Volt Switch. This effect is not stackable or reset by using this or another partial-trapping move.
Don't use this or any other low-power trapping moves. The only reason you would use it is for its effect, and even then, Mean Look or Block is almost always superior. In theory, Clamp could be used with Toxic to slowly stall opponents, and to keep them from switching and resetting the Toxic damage, although the Pokemon that learn it are much better offensively than defensively, and generally speaking you are unlikely to trap an opponent who has no way of hurting you, as few battlers would intentionally leave in such a Pokemon on your Pokemon.
Magma Storm is a stronger Fire-type alternative with 120 Base Power.
Bind is a Normal-type equivalent with lower Base Power.
Wrap is a Normal-type equivalent with lower Base Power.
Whirlpool is a special equivalent.
Fire Spin is a special Fire-type equivalent.
Sand Tomb is a Ground-type equivalent.