I learned heelflips. Proof:
https://www.instagram.com/p/BQQfr7_gKdl/ I skate 8 and below. This was a 7.6 deck. Anyway...
The trick to landing them, as far as I'm concerned, is where you balance/place your weight while standing on the board. You need to lean back and to the opposite direction to where your toes point. So it's like you're sort of edging that weight distribution a bit to that area. It's almost like leaning against a staircase, or chair on the back-diagonal. It's not a full lean, but just enough to establish that weight is being pushed towards that direction. So it's your back foot's heel position pocket of the board. When you do this and then do the trick, the board flips and you are flush with how it will rotate into the catch. One other thing I learned was that in the moment where you're actually doing the motion you need to have hyper-focus into what's going on. And final step is making sure you actually land with both feet on the board, which takes another little rush of adrenaline as well. I've been trying these for many years so to finally understand it was awesome for me. I posted in another thread how I learned this, figured I'd share my knowledge. I was landing them consistently for a bit, but some days it would be still hard to get one. It wasn't automatic every time.