I was trying to fold the top over on a tall, skinny doug-fir, near the chipper below. It was about 4" diameter and 20' tall. I was in the neighboring tree a handful of feet away, stripping as I went, or it was an interior tree in the grove and didn't have live branches below the top. 20' shorter would make the spar fit the yard. I cut the face at about 140*, way wide-open bird-mouth because of the location of the chipper.
That thing tipped right over, pulled the trunk of the tree forward Far before the face-cut closed and broke free. The rebound was 'something else'. You've seen a guitar string vibrate, well, this was about like that. Nothing to dampen the vibration, no climber's weight/ body on the spar. IF I had been closer to it, and it was oscillating back and forth, toward and away from me, rather than parallel to my tree, it would have beaten me, at the very least, senseless.