This mechanism gives the probe extra acceleration by stealing energy from a planet. When you throw a ball at a stationary object it bounces back to you at the same speed at which you threw it. However if you throw a ball at an object that is moving towards you then when the ball rebounds to you it does so at over twice the speed of your original throw. A gravitational interaction with a planet and space probe is just like bouncing a ball off it as the energy from the planet is transferred to the spaceship.
I didn't know that plants could give a boost to a planetary probe. Perhaps there are little known properties of chlorophyl that enhance the computer circuits aboard, or those gardeners who talk to their plants can send subliminal messages to any plants that may exist on the other planets.
(Don't take me serioulsy.)
My recently posted answer to this queation contained a typo, "serioulsy" for "seriously," which, now that I think of it, may be a clue to the oddity in the original quistion of "plants" giving a boost to planetary probes. Something to thimk about.
Alternatively Bruce Willis gives a concise scientific explanation in that classic flick Armeggedon. (Where the space shuttles use the moon to accelerate to 15G! You had to be there)