A character can use a Combat Maneuver or Power to force another character off a ledge. This must move the character into the space past the edge of the ledge. At that point both characters should roll a Reaction check. If the character that is being pushed off wins, they finish the move at the ledge, just dodging the fall. If the character that is pushing the other off wins, then the character falls. See the Athletics skill for details about falling.
The Reaction check is made whether it is a Move or Teleport with the results being the same.
This extra rule is in place to eliminate the ability to easily remove a character from game play. Everyone should be able to play Atomic all of the time. They might be at a disadvantage clinging to the cliff face 2 rounds down the ledge, but they are still in the game playing. This is much more fun. That is what the Atomic RPG System focuses on.