Unburden

 is a Pokémon ability introduced in the fourth generation. It increases speed once the user loses their item

Description
Pokémon with Unburden will have their Speed double when their held item is lost. If the Pokémon is switched out or was never holding an item, the ability will not activate. The speed boost is lost when the Pokémon gains an item.

Pokémon with Ability

 * Drifloon
 * Drifblim
 * Purrloin
 * Liepard
 * Hawlucha

As a Hidden Ability

 * Hitmonlee
 * Treecko
 * Grovyle
 * Sceptile
 * Accelgor
 * Swirlix
 * Slurpuff