its all in the properties of an elliptical orbit compared to a circular orbit. there is a *weak* point in an elliptical orbit where you can inject that satellite with little thrust relative to the magnetic poles of a heavenly object. compare that to a circular orbit where everywhere requires equal energy to get into its orbit.
you have anything harder than this?
to those who are unfamiliar with this topic, you can brush up on your analytic geometry and couple that with your astro-physics, and you'll get the drift.