While they have to both force in and out, the drain is usually cleared on the "sucking" stroke of the plunger. Air is compressible and you don't get much pressure one way or the other as you push down.
I have a plunger/force cup that has a vent hole up on the stick. You push down with the vent open and then block the vent hole. When you lift up on the stick, you can really feel the pressure and you almost always free a blockage on a push or 2. You don't get the action with a regular force cup that you do with this one but they both generally work the same way. With a regular force cup, the air gets squeezed out or compressed on the down stroke.