Hi this is taken from the documentary..
"When the fan blade fractures on the left engine, it's fragments damage the engine severely causing a reduction in fan speed. The a/t reacts by pumping more fuel to the damaged engine trying to maintain the fan speed. This causes the damaged engine to violently vibrate and throw out sparks, these ignite surface fuel and cause the flames seen by the pax."
"When the pilots d/c the a/t in order to shut down the right hand engine, both engines return to manual throttle. Sensors inside the damaged left engine detect that its spinning slower than normal which triggers a reduction in fuel flow. With the fuel flow now at a level the engine can handle the vibrations and flames disappear."
Hope this helps.
FS.