Wood grain is a funny thing, and most of us know that lumber contains all sorts of bound-up tension within it. On a good day, you'll cut a board, release those waiting forces and it will warp out of shape, ruining your project. On a bad day, you'll cut into it and the resulting forces will almost kill you, like this:
If you full-screen it, you can see the precise moment the crack develops. Talk about scary. You can see the poor guy doesn't even know which way to run, and prudently abandons his chainsaw.
What happened here is similar to what lumberjacks refer to as a tree pulling a "barber chair." Here's a more classic example:
The nickname comes from the classic barber's chair, whereby a man is made to recline in it for a shave; if you envision the act—the legs swinging up and the torso leaning back over the pivot point of the chair's central post—the moniker makes sense.
When a tree goes barber chair, obviously the potential lumber within it is ruined. But the more serious problem, of course, is the loss of human life. Even experienced arborists and lumberjacks have been killed by such incidents, and as you can imagine, the deaths are typically grisly.
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Canadian website Arboriculture explains what causes these incidents to happen:
…A barber chair occurs when using conventional back-cuts where the hinge is formed by cutting the wood from the back of the tree towards the hinge. As the saw severs the more resilient sapwood fibres typically found in the outer rings of a tree, the more brittle heartwood must resist the bending load. In cases of heavy forward lean and in older trees, this can result in the hinge wood splitting upwards as the tree falls. When the tree top contacts the ground the section of tree that has split upwards crushes either the remaining wood column straight backwards or the split standing section tears and rolls off to either side. In either case, the best place to be is away and at an angle.
If you look hard enough, you can find videos online of graphic barber chair accidents. We won't post them here because that's not our bag. But if you're an inexperienced DIY'er cutting a tree down in your backyard, please use caution, do your research, and/or consider hiring a pro.