I believe in the context of two strokes, over-rev is the ability to hold a gear and rev out higher, even if the engine is past the meat of the powerband. Like you're on a straight, and even though you should grab a gear, you don't because you need to slow/brake for an upcoming corner. Your motor will pull higher revs and you can keep accelerating, though not at the rate you would in the next gear. It depends on pipe, porting, bore/stroke, jetting etc. Many factors.
I could be completely off base though :-[:-[