In simple terms let's say you have a spacecraft and then you have a huge rubber band on the pinpoint of the nose. You pull back on the rubber band and point at the direction you want to go. At the point of your fingers the other end point of the rubber band where let's say your sling it, you have a powerful torch on that end of that pull. So while the torch is pushing the spacecraft you pull on the rubber band which, mind you, already is  going at full speed because of the torch. You fling the rubber band and it allows a relief or load off the craft, which in theory will give it an inertia while the torch is still pushing the craft. It will lighten the load immensely and it will travel even faster with the extra built up potential of the rubber band. Thank you.

Robert Brown