Clamp inflicts damage for 2-5 turns and has an Accuracy of 75. 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. Though technically only the first attack can critical hit, every attack during the duration will do the same amount of damage. While Clamp is in effect, the target will be unable to attack, and if the user of Clamp attacks before the target when used, the target will be unable to attack during that round as well.
Damage inflicted by Clamp's continuing duration is done after recurrent damage. If the user switches out before Clamp's duration ends, the target will be unable to attack during that turn. If the target switches out before Clamp's 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, but the current PP of Clamp will roll over to 63, and full PP Ups will be applied to it.
In Pokémon Stadium, if the target Pokémon switches out before the duration ends Clamp is ended and the user's turn is skipped.
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. As said above, 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.
In-game, the target will get to select a move during each turn of Clamp's duration, and will attack your incoming Pokémon with the selected move if the player decides to switch before the duration is over.
In Stadium, the player selects a move 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.
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>!".
In Generation IV, if the user of Clamp is holding a Grip Claw, the duration will always be 5 turns. Also, if an affected Pokémon is holding a Shed Shell, it can switch out.
Clamp's accuracy is increased to 85, and 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.