To do this you need Hydros Level 2 or 3, I havent tried it with level 1 yet.

But you need to be faced up against a wall strait ahead or at a 45 degree angle. Press the E-Brake button and start moving your hydros Up and Down as quickly as possible. You should launch yourself up into the air...

Below are some pics I took to show yall.

Sorry for the detail of the pictures... Something is up with my Digital Camera...

 

cool i'll try it

 

lol i never seen that before i wasnt even aware u can do that

 

Are you on top of the building???

 

no im not on the building lol... just a wierd glitch in the game to where your car can hop in the air