Power Band is an item introduced in Generation IV that adds 4 Special Defense EV points to the Pokémon holding it when it gains Experience Points, regardless of the EV point(s) it normally gains from the opposing Pokémon. While it is held, it reduces the Pokémon's Speed stat by half.
When held in Pokémon Mystery Dungeon games it will raise the attack stat of the Pokémon holding it.