The Slingshot Effect

The slingshot effect has its origins in astrophysics.  It is a term used to describe how a spacecraft can utilize the gravity of a planet to propel itself forward at an accelerated rate.
Imagine shooting a marble at a bowling ball that is rolling slowly toward you.  The marble would absorb the momentum of the bowling ball and bounce off it at an increased speed. The bowling ball would even slow down by some small amount.