>If a Pokémon with Truant uses a move such as Hyper Beam, which recharges after use, then Truant will take effect during the turn that would normally be used recharging; however, the Pokémon still cannot switch out -Bulbapedia
So Slaking would use Hyper Beam, and then Traunt would take affect while it's recharging, but you can't switch.