Start off kicking the deck with your toes like a tailwhip but the other way and move your wrists in a circle like you would for a tailwhip. Practice it over and over untill you got those dialed and then when you start learning heelwhip catches, if you are like me and catch with both feet, dip your toes down a little bit and kind of pull the deck up when its under you.
unlike tailwhips, its all in the hands, so kick as hard as they can and whip really hard, jump higher then you think you have to, so you whip under you. done