Simple way to imagine this in your head.

Hold two ping pong balls at arms' length, one above and further away than the other. Right now you are the sun.

Put a red dot on top of the lower one, and then rotate it just slightly until you can't see it. This is the observer on Earth, who is after sunset.

Put a red dot directly in the middle of what you can see on the upper ball. This is the center of where the sun is striking the moon.

Keep looking at this dot on the moon. Now, keeping their relative positions fixed, bring the balls towards you and up, until you are looking from the perspective of the observer on Earth. What happens to the dot on the moon? It appears to rise up away from you.

That's the paradox.