You can do more than use planets to speed you on your way. If they are right next to each other, (or just one space away), you can transport people from one to the other endlessly.
Seems to me that entering or leaving a planet should take up one movement point, but... Oh Well.
What you are doing, instead of landing the ship, is using the planet's gravity well to sling-shot your way past it. Using a planet in this way doesn't require the engine to do the work of actually moving the ship as the planet is taking over that part of the work. And so you sorta get a free movement point for doing so. Or at least this is what I believe happens.