Every second turn, a Pokémon with Truant cannot use a move due to loafing around, such that the Pokémon can move on its first turn but not its second turn. Switching out or gaining the Ability Truant resets the turn count for Truant. Using an item does not count as a turn for the purposes of Truant. Truant is reset by sleep and a Pokémon with Truant will always be able to attack on the turn it wakes up.
Moves with a charging turn, such as Fly, will not work (unless the user is holding a Power Herb), because the turn in which the move would be executed is the same turn in which the Pokémon will be truant. If the Pokémon uses a move that requires recharging, such as Hyper Beam, then the Pokémon will loaf around due to Truant during the turn that would normally be used recharging; however, the Pokémon would still recharge as such and thus would not be able to use items, switch out etc.
If a Pokémon gains Truant after it Mega Evolves but before it moves, Truant will activate on its first attempt to move.
In Pokémon Super Mystery Dungeon, Truant instead causes the truant status condition for one turn. Unlike the previous Paused status, the truant status only prevents the Pokémon from using moves and allows the use of items. Truant additionally can inflict the Yawning status condition on all surrounding enemies when the Pokémon with the Ability Truant is asleep.