Complex sentence: When the airframe sees fuel disappearing from its tanks and feels a thrust the understanding of how a particular fuel flow produces a definite amount of thrust at a particular point in the flight envelope is called jet engine performance.
Split into simpler sentences: When the airframe sees fuel disappearing from its tanks and feels a thrust. The understanding of how a particular fuel flow produces a definite amount of thrust at a particular point in the flight envelope is called jet engine performance.