you said on your title, tree
stumps. They are not easy to split out with any system manual or machine. A straight clear section like that in the picture will almost split itself once its started as Robin described. I just split up (by hand) over 3 tons of willow, basically the healthy remains of an entire mature treee, 4 foot across at the base. Most of it was easy to do, but the base sections were not. The grain runs diagonal, this way that way knitted together in all directions depending on where the root system joined the trunk of the tree. In the end I chainsawed it into smaller sections and then went at it with a heavy splitting maul, but it took WAY longer than normal clear logs..... LOL there is a firewood wholesaler across the road from here (didnt buy the willow from him), I cant imagine why theres a mountain of rooty sections at one end of the yard. I dont think even their 30+ton splitter can break them down...
One tip, if you have a "Y" section, your instinct is to aim the axe in the vee. But the grain is attempting to prevent the Y of the branches from breaking apart-as it grows when its living, and now the same as you try to split it. So, try this-turn the wood 90 degrees so your looking at the side of the Y, not its front, and split that way, so you get 2 half Y's so to speak. It always works. Cheers Jonathan