I was going to mention the knee-bending answer (listed above in purple - sorry, can't remember poster!) - an actor told that one to me.

But....I was also going to mention (and now I am mentioning it!) that there's another possibility: that "leg" is short for "legend" - hence, "break a legend" - ie, "be really, really good!"

Or as Bill and Ted of Bill and Ted's Excellent Adventure would say: Be excellent.