Bruz, the quick and dirty version goes like this:
1. Heat the billet and hot chisel a line where you intend to split it, about halfway through the stock. A bit more is better than a bit less. Normally you'd just hot-cut through the whole thing, but that leaves the sharp edge that will cause cold shuts if you leave it on there, so it requires laborious filing if you're unplugged. With a grinder it's faster and easier, but still not as high on the "Neat" scale as the water technique.
2. Keep the steel hot or, more likely, reheat it. Place it in the vise with the cut line just above the jaws and bend it along the cut line. I believe the advice is to bend it toward the side you chiseled, but I'm not sure that's necessary. It does seem to make the cut more symmetrical along the edge.
3. Get the steel good and hot, and pour lukewarm or cold water along the "outside" of the cut (if the bent piece of steel is considered a V shape, you would want the point of the "V" to be up and would pour the water along the edge. Honestly, though I haven't had a problem, I think cracking is a concern when using water to quench something like 5160. I like to hold the steel so the point of the V is on the side so I can pour the water so that it only touches that outermost edge of the bend. The kamis use teapots, and they seem to be the best way to get a controlled stream of water that's high enough volume to cool the steel rapidly.
4. You can twist the two pieces apart, but they'll actually crack cleanly right down the channel you hot-cut most of the time. It's a nice clean cut and requires very little filing or cleanup before going right back to the forge.
If this was unclear, I'm not terribly surprised, but HI does sell a video on their website that shows the process. It's not really a tutorial on how to do it, but it's one of those things. You see it once and think "That's so simple! Why did I do it any other way?"
. They have a standing order at half the junk yards in India for the springs out of crashed Mercedes trucks 
