In the UK, High key is called at the point when you are turning cross wind, generally on the deadside. Low key is called downwind at the point you are turning base.
However, in the UK these calls are not made in standard overhead joins, but generally used in the case of practice forced landings to land from the overhead.
High and low key are not standard civilian RT/Brevity expressions. I believe that they are military in origin, that have crept into common GA useage.
