To burn into lunar orbit, the spacecraft first swung around the moon, all under the influence of the moon’s gravity.

If for some reason the rocket engine had not burned, Apollo 8 would have swung around and returned to Earth. It is on a Free Return Trajectory.

On the backside, once they reach the closest approach to the moon, the engine fired for 250 seconds to slow them down enough to be captured by the moon.

Apollo 8 remained in lunar orbit for just over 20 hours, long enough to orbit the moon 10 times.