When making the back-cut on a tree, the tree can quickly split up the center kicks out. This usually results in decapitating the cutter or at least sending him flying.
A small flat spot with a back is what is left of the stump. The tree is hanging over the cutter's head.
See also Bight of the Line, High Rigger and Tree Harvester.
This bit of jargon has been viewed 8,077 times.