Basically, a Gardevoir is sent out and traces a Regenerator Slowbro. Gardevoir is hit by water pulse (50% for example), Gardevoir attacks with energy ball, Gardevoir is taken out. When Gardevoir comes out will it have regained health?
This effect will occur if Regenerator is the Pokémon's Ability in battle when it switches out and is not suppressed; a Pokémon that obtained the Ability with Trace will restore HP when it switches out, whereas a Pokémon which has Regenerator but has been affected by Gastro Acid will not. Heal Block does not prevent Regenerator from restoring HP.
The Traced ability is removed upon switching out, but Regenerator will activate before it's removed, so the health will still be recovered.
Source
Pokémon that obtained the Ability with Trace will restore HP when it switches out.
