In the description, he says that the tree was leaning hard over the house. So the pull line is sideway, as it seems for me on the vid, with not so m1uch force. The main motor is only the top's weight, maximised by the heavy lean.
I guess that he made just an undercut instead of a face cut, then the back cut acted as a snap cut, releasing the top at max speed. Problem, he didn't have the time to finish the cut and a wood strap remained on the side. By this strap, the top gave an hard pull on the trunk. Finally, it teared the wood under the cut, releasing the trunk at full flexion. Physicaly, the climber can't follow the movement. That's very hard for the back, I have two bad recalls with that. Not such a ride though.
I didn't know that a chainsaw could fly!
R.I.P. the ms200T.