Just a quick safety note on the video from Timothy: when working on a bigger log, I would never put my hands into the split while still under tension like this bodger does at 1:13. If the froe slipped back into line, the log would really pinch. On smaller pieces, it won't really be a problem.
Once the split is complete enough that there's no more tension, I often reach into the split to pull the halves apart to snap any fibres that are still sticking them together.
In my experience, controlling the split is tricky. Probably a matter of practice.
Unless it's a really short piece, you'd almost always do it in halves.
The idea is to make the thick side bend away from the split more. The way I picture it is that this will stress the fibres on that inside surface of the split and make them peal off, staying with the thin side. Don't know how accurate that is.
Here's a video describing that:
I think with a thick piece where sticking your hands into the split isn't such a good idea, the same kind of differential pressure is applied by which side you put up and which side down in the brake, since the two trunks of the fork touch at different points along the piece you're splitting, and you can lean on the bottom half with the froe in addition to twisting in.
See this article, too.
Give me a brake - Peter Follansbee
I was recently taught a bit of spruce root basketry, and that really clarified the concept on an intuitive level. Dig up some spruce roots, scrape the bark off with a knife or a tough thumbnail, and try splitting them. The roots are much more flexible and easy to bend around than wood, so perhaps easier to grasp the theory with. (Then go sew something with them.)