Smack Down inflicts damage. If the target is airborne, Smack Down forces it onto the ground, removing the effects of Magnet Rise and Telekinesis. Smack Down makes its target vulnerable to Ground-type moves and Arena Trap even if it is Flying-type or has Levitate. This effect lasts until the affected Pokémon switches out.
This effect does not occur if the target is behind a Substitute or has temporarily grounded itself using Roost (even in subsequent turns when the Pokémon returns to the air).
Smack Down can hit a target that is using Fly or Bounce and knock it to the ground (canceling the move). It can also hit a Pokémon that is using or is affected by Sky Drop, but can't knock it down. A Pokémon hit with Smack Down can still use Fly or Bounce on subsequent turns.-Not bulbapedia.
The effect on flying types is that they lose their immunity to ground type attacks.
The point of the move smack down is to remove the effect of levitate and such.