In Physics of the Ollie you learned about how the rider becomes airborne.
Understanding these tricks from a scientific perspective would require understanding principles of
friction
gravity
motion
acceleration.
To start the board flipping, the rider kicks down with his/her back foot, and forward towards the nose with the other foot.
This starts the spinning.
But why, after the rider kicks the board, you might ask, does the skateboard keep flipping. The answer lies in a mystery that Newton solve years ago.
Objects like to be lazy, and you'll find that it takes less work for the board to keep flipping than to stop.
Objects in motion like to stay in motion.
After the board has completed it's flipping motion, the rider stops it with his/her feet and stomps down.
Then the rider rides away clean.