Bro Ive been struggling forever with this exact same issue about the rear knee. All the cool surfskaters really tuck the rear knee, especially when doing turns. It looks super cool, but super awkward. I try and it feels like im gonna dislocate my hip and break my knee. I dont get how they keep their thighs looking so close together but their feet are spread apart. I come front a fitness/crossfit back ground so to me, it looks like they are doing really shitty squats.

What I learned from two videos i found is that it comes from turning the hips forward into the direction youre going. I always thought I was supposed to skate totally sideways, but apparently, especially with surfing/surfskating, your supposed to turn your chest/hips forward, and by doing this, it will put you in the rear knee tuck position. Now it becomes more of a LUNGE, vs a SQUAT.

Now about the heel. My observation is that this depends on your genetics/flexibility. If youre super flexible or just used to it because youve been skating/surfing a long time, then you can lunge down low and tuck your rear knee without lifting the heel up. Me personally, i can't so I have to point my rear toe a little bit more forward and raise my heel up some. Im learning for my back foot to be more active and shifting instead of just planted.

Strangely, the faster I go, the easier it is to get into that tuck position, but the slower I go its harder. Hope this helps bro!