I've always found heelflips much easier than kickflips, so I spent a good 6 months focussing on kickflips. Consequently, I lost heelflips. In the re-learning process, I've noticed the one big difference for me between the timing of a kickflip and a heelflip is the flick with the front foot needs to be much earlier with a heel than with a kickflip.
That being said, my foot placement looks nothing like these other guys. Isn't it weird how such a massive difference in foot position, weight distribution and movement between different people can end in the same result? I love you, skateboarding.