I believe that they normally just bore a hole near the center of the log, pack it full of gunpowder (use a hammer to tamp it down, like you would tamp the powder in a muzzle loader) insert wick, plug the hole very tight with dirt -this is important, as black powder works better the more compresses and confined it is. Then light the wick and run away.
Black powder does not respond well to an electrical fuse, it takes a live spark to ignite it. Flash powder does, but is not nearly as explosive and is more expensive. The safest bet may be to have a flash powder primer set of by a remote switch, but it would be easier to set up a 20 second fuse and take cover.
I suspect that in most cases the split will happen so fast -in a tiny fraction of a second- that it would be difficult to direct it very much.
With this method, the log is split by the intense pressure generated by the burning black powder (which as I understand it, is not technically an explosion), the gasses build up so fast that they have no where to go, and so they force the two parts of the log to fly in opposite directions, splitting it quite dramatically. For this to work, the charge needs to be as close to the center of the tree -both length and depth- as you can manage to get it.
I have heard that as little as 5 ounces of black powder is enough to split big logs, for logs this size thought I may want much more. The last thing I want to do is blow a charge that is too small, all that will do is cause shock to the wood which is bad.
We all out here are a bunch of hicks anyhow, so this kind of thing is like what we do for sport! Haha..
