Whenever the Pokémon holding it deals damage, it restores 1/8 of the damage dealt as HP. This means that it’s quite good for Pokémon which do a lot of damage. I would not recommend using it if you have Leftovers, because Leftovers heal you even if you don’t do damage.
As usual for the Pokémon Item in the Spotlight, this is quite a short post! Next’s weeks one will be longer!