To "upstage" someone, you would move upstage, that is, move up the rake
(towards the back wall)-- If you got the other actor
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.