The slightly kinked diagonal line on the main part of the map is known as the surge (or stall) line. Above this line is a region of unstable flow, which is an area best avoided.
A compressor surge or compressor stall causes an abrupt reversal of airflow in the compressor. Compressor blades create a pumping action by working as airfoils. In a surge or stall, blades experience an aerodynamic stall (similar to an aircraft wing stalling) and become unable to hold back the higher pressure downstream, with a resulting violent flow reversal. The flame which is normally confined in the combustion chamber may come out of the engine inlet as well as the exhaust nozzle.