It is the way the engine sounds when running. This is usually referenced in the break-in section of engine manuals. Taking a 2 cycle to a 4 cycle note means turning the high-speed need out (richening the mix) to the point where the exhaust note changes due to lower rpms and a richer mix. Taking it back to 2 cycle would be leaning the high speed back to the normal position where the engine spools up again and sounds normal.