To "upstage" someone, you would move upstage, that is, move up the
raked
stage (towards the back wall)-- If you got the other
actors
downstage of you to then face you, away from the
audience, and the
attention of both the actors and the audience are now on you.
The term now refers to any means of drawing attention away from someone else. You could upstage someone by moving in front of them, falling down, sneezing, mugging, taking off your clothes, or just being more talented than them.