To do a heel flip, put your feet in the same position as in an kick flip, but your leading foot should not be hanging off the board as much. Your toes should be lined up with the toe side of the board. Pop an ollie and as you slide your leading foot forward kick it off your board to the toe side. This should make the board spin the opposite way of a kick flip. The important thing is that you have to get your leading foot out of the way. Then, when it spins halfway, bring it back, and as the board finishes spinning smack it down with both feet.
Tips: Whoever named this trick must've been a moron or stoned or something. This trick has nothing to do with the heel.

Trick contributed by: www.skateboarduk.com