The Wood Grenade is a diamond section wedge, designed to split firewood when used with a sledgehammer.
It works pretty well, it's heavy and pointed enough to be stuck into the log by hand then you take a step back and whack it with the big hammer.
I used mine today to split up some large beech which I'd left too long and my maul was just getting stuck in. Sometimes the rounds were splitting into 4 pieces, usually 3.
I think it would be too slow to place and use on fresh or easily split wood, the maul is the tool for that, but if you have older / tougher rounds then it's a useful tool.
Costs about £6 .
Pic here; http://undercrank.com/illustrations/wood-grenade.jpg