Tony Hawke and Eric Koston both define a hard flip and then demonstrate it. It is a flip of the skateboard while going on a flat surface, like a parking lot. Push down with your back foot, apply pressure on one side, then get your feet out of the way (by jumping) to allow the board to flip, then land back on the board (all while moving, of course). Apply a quick hard turn at the end to compensate for over-rotation. Tony and Eric joke around about Tony's inferior hard flips but he's a vert skater, after all. This jump can be done regular or goofy footed, and Eric prefers regular.
Start your career in Graphic Design with the WonderHowTo's Beginners’s Guide to Photoshop Course