Clamp inflicts damage and traps the target for 2-5 turns, damaging the target at the end of each turn. There is a 37.5% chance that it will attack for 2 turns, a 37.5% chance that it will attack for 3 turns, a 12.5% chance that it will attack for 4 turns, and a 12.5% chance that it will attack for 5 turns. Although only the first hit can be a critical hit, each consecutive turn will deal the same amount of damage. Whilst the target is trapped, it will be unable to attack, and if the user of Clamp strikes before the target, when used, the target will also be unable to attack during the same turn.
Damage inflicted by Clamp's continuing duration is done after recurrent damage. If the user switches out before the turn duration ends, the target will be unable to attack during that turn. If the target switches out before the turn duration ends, Clamp will automatically be used against the incoming Pokémon, deducting an additional PP from the move. If at such a time Clamp has 0 PP, Clamp will still be used against the incoming Pokémon. After that use, the current PP of Clamp will roll over to 63, and full PP Ups will be applied to it.
Even if Clamp misses, it will negate the recharge turn normally required for Hyper Beam. Additionally, if the user of Clamp attacks before the user of Hyper Beam during a recharge turn and the use of Clamp misses, the user of Hyper Beam will automatically use Hyper Beam during that turn. If at such a time Hyper Beam has 0 PP, Hyper Beam will still be used, and afterwards its current PP will roll over to 63, and full PP ups will be applied to it.
The target will be able to select a move during each turn of Clamp's duration, and will hit an incoming Pokémon with the selected move if the user switches out before the duration is over.
In Pokémon Stadium, if the target switches out before Clamp's duration ends, the move is ended and the user's turn is skipped. A move can be selected during each turn of Clamp's duration. If the target switches out before the duration ends, the incoming Pokémon will not automatically be attacked. Clamp will negate the recharge turn of Hyper Beam only if successful.
The target is now able to attack during Clamp's duration, and can act normally. Instead, it inflicts 1/16 of the target's maximum HP as damage for two to five turns upon use, in addition to the damage dealt when it is used. "<Pokémon> was CLAMPED by <User>!" will appear. It also traps the target, preventing switching and escape. If a trapped Pokémon uses Rapid Spin, it will be freed.
Generations III and IV
The effects of Clamp are the same as in Generation II, but if a wild Pokémon uses Clamp on the player's Pokémon, the player may escape if the affected Pokémon has Run Away or is holding a Smoke Ball. However, these do not allow the player to switch the Pokémon out. The text also changes to "<User> CLAMPED <Pokémon>!"
If the user of Clamp is holding a Grip Claw, the duration will always be five turns. Also, if an affected Pokémon is holding a Shed Shell, it can switch out.
Clamp can also be used as part of a Pokémon Contest combination, with the user gaining extra three appeal points if the move Rain Dance was used in the prior turn.
Clamp's accuracy is increased to 85, its PP is increased to 15, and it lasts four to five turns. If a Binding Band is held by the user, the damage done at the end of each turn will increase from 1/16 of the target's maximum HP to 1/8.
The end turn damage of Clamp is increased from 1/16 to 1/8 of the target's maximum HP. If the user is holding a Binding Band, the end turn damage of Clamp will increase to 1/6 of the target's maximum HP. Ghost-type Pokémon cannot be trapped by Clamp.