Flaps, a jet engine works by progressively building up air pressure as you go backwards in the engine. Located along the engine are a few valves which can release pressure when it builds up too much- these are called bleed valves. If they get a bit sticky, or the engine design is not quite right, air pressure can build up in certain areas and be too great to be restrained- sometimes this pressure will spill out forwards. So the engine does a 'cough', sometimes quite violent and throws out air forwards. It is accompanied by a loud 'boom' or sound of canon firing, great vibration, and sometimes startling indications of exhaust gas temperature rise- the steady state burning has been interrupted and the engine may not be working at the right speed for the air coming through. This can manifest itself as either EGT staying fairly stable after a rise, or rapidly running towards limits requiring a quick shutdown.
The VC10 of old ws prone to surging at high altitude- the junior cabin crew working in the rear galley would be a few feet away from a canon going off! You could count to 10 after one and a wide eyed junior crew would breathlessly burst onto the flight deck believing a bomb had gone off it was that bad! The 747 would sometimes not recover itself and need to be shut down. I have sat next to a 707 engine at night continually surging and it was spectacular, bangs and a continual firework display with sparks flying forward in the dark (complete hydraulics failure on the ground requiring power to stop rolling off the runway).
The amazing thing is, despite such a savage event, I don't think engines fall apart because of them- they can recover and run on quite happily in the majority of occasions.