Breaking the Plane – players define where their characters are in relation to each other by choosing where to “look” for that character.
For example, in a baseball scene, rather than throwing the ball across the stage to each other, Player A throws the ball toward the audience, as if Player B is in that direction from Player A. Player B, standing parallel to player A, then receives the ball from the audience.
Another example is a scene in which a woman looks out her bedroom window and talks to a man on the street below. Rather than trying to convey this physical scenario while looking at each other across the (level) stage, the woman faces out to the audience and looks downward as she talks (as if the man is in a hole in the stage), and the man faces the audience but looks up as he speaks, as if the woman is in the ceiling of the theater. Similarly, an improviser can watch his fellow player go up a tall winding staircase by watching the ascent while the ascending player is really standing on the same level stage.
Rather than be confined to a set-less stage, “breaking the plane” allows improvisers to create a more interesting stage picture.