First get close to a car until the hijack button appears, then hold your left mouse button (headbutt button) until you are fully charged up, and then really quickly let go of the mouse button you were holding and get in the car (get in the car right after you let go of the mouse button) and there you go you are now flying
(you might not get this first try)


How it works: since you are headbutting and letting go of the button the game thinks you are still headbutting so it just happens inside the car lol..

NOTICE
THIS WORKS BETTER IF YOU STAND ON A CARS ROOF IF YOU DONT WANT TO FLY BUT JUST SPEED ON THE ROAD