I think it's quite easy to picture actually:
Imagine yourself in the car (in the backseat) and throwing a ball forwards, you're suddenly throwing objects in excess of 80mph!
The absolute velocity of the ball is not 0 when you throw it, it's 80 mph. Your power is just adding to that velocity.
Or look at it this way: The fly can fly because it pushes against the air around it right?
That air appears stationary in the car, because it has nowhere to go (compare to a convertible). So the air moves in exactly the same way as the car. The fly pushes against that air, and so it's moving at a normal air-speed, but airspeed + 80mph = absolute speed.
And if it still puzzles: the earth is revolving (at quite high speeds around the equator). But people can walk easily in any direction. It is because we walk relative to our surroundings (which just happen to go very fast through space).
Note: I was told once (don't know if I remember correctly) that if the earth would abrubtly stop spinning we would fly into the wall at an average of 350 mph!
Edit: I hope I'm making sense it's 6 am here
