We often here of star systems loosing a planet or objects (satellites) getting accelerated away from a huge source of gravity (huge mass).
In: Physics
You’re probably thinking of a [Gravity slingshot](https://en.wikipedia.org/wiki/Gravity_assist). It only works because the planet (or whatever) is in motion. Thrusting while closer to a planet as it’s passing you has a bonus effect as opposed to thrusting in empty deep space. And the planet slows down a tiny fraction.
Likewise if you wanted to fly into a solar system and slow down to match speeds you’d thrust while Jupiter is coming towards you on it’s orbit path. It can work as a space brake too.
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