Jon, yes, that's it. You can use these across a spectrum of tension techniques. I use small amounts of tension, basically just in sync with my natural breathing pattern, in the mornings. It's not just pushing down through the heel - there's a long list of ways to work on that position, with some attention to the bent leg and some to the spine as well as to the straight leg. Think of the cossack as a one-legged goblet squat - pry with the elbow inside the bent knee. Focus on going a little further as you exhale. Flex and extend the ankle, both positions of tension, and leave the foot in neutral as you exhale.
You also asked about splits:
After you've done the above cossack sequence, the next sequence is to work on martials arts splits to each side followed by a side split.
Waiting out the tension technique:
In the mornings as part of a warmup, you might wait out the tension but only briefly. It functions as a sort of gut check - do you own the position, can you relax in it? Not much contraction, and only a short relax.
On the other hand, if this is a serious session at the end of the day, go through several cycles of contract-relax and, when you're as far as you can go, stay there for a while.
Here's a good technique to try: use pillows (sofa cushions work great, ideally something not too soft) to raise your hips off the ground enough that you are in a perfect side split - perfect in every way except depth. That means your feet and hips will be in a line, not a V, and you'll be able to arch your lumbar (or at the very least keep it neutral, and certainly not rounded). Important: Use whatever support/insurance you need, e.g., do it in front of a wall or a sofa if you're worried about falling backwards, put a chair or low table in front of you - do whatever you need to feel comfortable and safe.
Then wait out the tension there - it's a different experience altogether to have your lumbar spine right where it should be, and you want to be sure to get high enough off the ground that you can do this. Watch a half-hour TV show, read a section of the newspaper or a short story - that kind of time. Get comfortable.
20-30 minutes is good - start off with 2-3 minutes and work up, and always remember that it will feel weird getting out of the split - that's perfectly normal and expected, so take your time.
If you do that once or twice a week, combined with other splits practice, you'll find that, over time, you look at that pile of sofa cushions and think about trying the whole thing just a little closer to the ground, and that's how you progress. You can even remove a cushion during a single session but it's best to have an assistant (in my case, my kind and generous wife) to do that for you.
Hope that helps, let me know.
-S-
