I apologize for jumping in, but I am also trying to learn to post. It sounds like you are not supposed to tighten up your legs when you post. I understand the part about letting the horse's bounce toss you up a bit, but I do not really get how I am supposed to make the descent gentle instead of thumpy...
Some of it depends on your goal. If you jump horses, or want to, then someone else will have to answer. For general purpose riding, you rhythmically put some weight in the stirrups to slow the descent. The more weight in your stirrups, the less in your rear when your rear contacts the saddle.
Momentum is my enemy in this. If I post high, then I have a longer way to come down, and I have to apply more pressure to keep from falling into the saddle. That is why I like a minimal post - the less I go up, the less I come down. If I have a shorter distance to come down, I'm less likely to PLOP
into the saddle because my rear doesn't have a chance to build up speed. It also allows me to post faster, since there is less motion between beats. The shorter the distance my butt travels, the sooner it can complete the trip.
Without stirrups - I'm a beginner and have only done this a couple of times. I do practice at times putting the least weight I can in the stirrups. That requires some squeezing with the legs (preferably the thighs) to provide some friction to slow the descent down. But I haven't tried it enough to give advice on how best to do it.
Also, heels down is good, but it isn't the end all of riding. As my feet approach the line of my hip, it becomes almost physically impossible to get my heels past level. Since I normally ride in a mild chair seat, that isn't usually a problem. A forward leg will help lower the heel, but you are fighting gravity any time your stirrups aren't hanging straight down.
If I try to force my heels down, I put a LOT of tension in my leg, start bouncing, and things go downhill fast. A relaxed leg (not a sleeping leg - some tension is good) is more important than lowering the heel another 1/4".
All advice from someone in the learning stage himself...