If you are only heaving someone bodily out of an adjacent square I'd call it a Bull Rush attempt, straight up. You roll STR vs. FORT to push the target 1 square and shift into his vacated space.
Anything else you attempt would be a Stunt. There are no hard mechanics for it so it would be handled at the discretion of the Dungeon Master.
If I were the DM I'd make it take a Standard Action that you could only take against a target you are Grabbing - and it would cost you an Action Point. Then I'd make it an Athletics Check with a base DC of the subject's Fortitude Defense. Further, I'd allow you to
slide the victim rather than push him, to allow for lateral throws and the like.
- If you succede on the check you may slide the victim 1 square and knock him prone.
- For each 5 full points by which your check result excedes the DC, you may slide the victim an additional square.
- For every 2 full squares the victim moves he suffers 1d10 falling damage.
That's just a quick-and-dirty improvization on my part as a DM, though. I'd make it cost an action-point because the desired effect is rather significant but it is definitely dramatically appropriate. This way you can pull off some fun and effective tactical improve without opening the door for all the PCs and monsters to start spamming some sort of unplaytested Judo Rules every round.
- Marty Lund