Complex sentence: This has the effect of giving the rocket a lot of thrust initially, accelerating it up to high speed rapidly, and then once all the fast-burning propellant has burnt, the slow-burning propellant begins delivering a much lower level of thrust.
Split into simpler sentences: This has the effect of giving the rocket a lot of thrust initially, accelerating it rapidly to high speed. When all the fast-burning propellant has burnt, the slow-burning propellant delivers a much lower level of thrust.