Here’s an analogy. Two flies smell a dumpster on the other side of the playground. A race ensues.
The first fly gets a head start. He sees a stationary basketball on the ground. He decides to slingshot around its gravitational field. As he dips into the field, he gains velocity. It’s working! But, as expected, the energy needed to escape the ball’s field slows him down again. Damn!
The second fly sees Napolean Dynamite playing tether ball. He makes some quick calculations. He determines that if he approaches with just the right timing, velocity and trajectory, he can dip into the ball’s gravitational field while it is in orbit. High risk, high reward.
He times his arrival perfectly. As he dips into the field, he gains velocity from the orbit while the gravitational field keeps him with the ball. As he escape’s the ball’s gravity, he shoots out across the playground and splatters into the side of the dumpster. Victory!