Hi Alfred,
You're right; my right hand mainly controls angle and stone contact. My left hand mainly controls pressure. The left hand fingers also keep the bevel contact with the stone, yes. I take several seconds to position my right hand fingers on the handle to loosen my fingers up and get a comfortable grip.
As for the rocking, it is inevitable, and every freehand edge will have a bit of convex to it. That said, the way to really keep an even bevel when freehand sharpening is to follow it in your sharpening strokes. Using your body to "sway" to make your strokes allows you to lock your wrists and fingers, lessening the amount of "rocking" you do with your hands/wrists. With practice, you will get more consistent, even, "pretty" bevels that are the hallmark of good stonework.
In the below vid, knifenut does sharpening passes for a few minutes one-handed to illustrate the importance of finding and following your bevel (watch 01:05 - 02:40). He then explains that you need the other hand for two reasons: A) to apply the pressure you need for the grinding you are doing (you can't do it with one hand), and B) to keep pressure contact points on the blade if it's a flexible blade, otherwise the blade will flex on the stone and not sharpen evenly along the whole bevel (watch 02:40 - 04:20). This is very, very important for a longer, more flexible blade as it becomes entirely a pressure and contact game.
However, even a slight bit of rocking isn't really an issue as long as you are hitting your apex evenly along the edge, which is the whole point of sharpening: forming a clean apex (and deburring, of course).
[youtube]XgOXvtJDm5E[/youtube]
However, for a knife as thick and hard as this LionSpy, the contact points are not an issue. The left hand just controls pressure (and adds steadiness, regulating contact). You can keep your fingers at any point that's comfortable to guide the bevel along the stone and keep even pressure.
I'm not a pro, and I'm nowhere near the skill level of guys like knifenut or Murray Carter (they just have so much experience and technique), but I enjoy the art of sharpening, and I'm always improving. And I think making vids is fun, so I'm happy to share what I do know so we can all learn together.