Many people assume that when one bids a performer, "Break a leg!" that it's a kind of a reverse jinx, and that they are really wishing that they don't break a leg, and that all goes well. Actually the expression comes from the pre-movie era in the 18th and 19th century where a performer doing an exceptional job in a scene in a play would occasionally stop the show for a moment as the audience broke into appreciative applause for his or her performance. The actor or actress would then modestly come forward to the footlights and acknowledge the ovation with a bow or curtsey, in other words, bending the knee, or "breaking the leg." Once the audience's demand was met, the performer would get back in character, rejoin the other actors in the scene and the play would continue.
So when one wishes a performer to "Break a leg" one is actually saying, "Do so well that you stop the show and have to take a bow!"