Whenever the Pokémon holding it deals damage with a move, it restores HP equal to 1/8 of the damage dealt (rounded down, but not less than 1). It will not restore HP if the move inflicts no damage (e.g. using False Swipe on a Pokémon with 1 HP).
If the holder uses a multi-strike move, it recovers HP after each strike, considering the damage from that strike only. Future Sight and Doom Desire do not activate the Shell Bell. The Shell Bell activates even if the move only hits a Pokémon's substitute.
If a Pokémon steals a Shell Bell with Thief or Covet, it will recover HP for the use of that move that stole the Shell Bell. The Shell Bell activates after the holder recovers HP with a draining move or takes damage from recoil and Rough Skin.
Generation IV
If the holder uses a multi-strike move, it recovers HP after the last strike, considering the damage from all of the strikes at once. The Shell Bell will not activate if the move only hits a Pokémon's substitute. The Shell Bell can heal a Pokémon regardless of Heal Block.
Generation V onward
The Shell Bell activates even if the move only hits a Pokémon's substitute. The Shell Bell cannot heal a Pokémon affected by Heal Block.
If a Pokémon steals a Shell Bell with Magician, it will recover HP for the use of that move that stole the Shell Bell. The Shell Bell activates before Pickpocket, so even if a Pokémon's held Shell Bell is stolen by Pickpocket it still restores HP for that move. The Shell Bell activates after the holder takes damage from Iron Barbs and the Rocky Helmet. Shell Bell's recovery is skipped if the move was boosted by Sheer Force.
In Pokémon: Magikarp Jump, Shell Bell is a Friendship Item that can be purchased for 500. It summons Greninja as a helper Pokémon.
Pokémon UNITE
Shell Bell in Pokémon UNITE
In Pokémon UNITE, Shell Bell is a held item that makes the user restore user's HP when landing move, with a 10s cooldown, while also increasing their Sp. Atk and CD Reduction. Each hit recovers HP equivalent to 45/60/75 + 35/40/45% of user's Sp. Attack at Grades 1, 10, and 20 respectively. At Grade 1, this item increases Sp. Attack by 1.6 and reduces CD by 0%, and by 24 and 4.5% at 30.