Maybe I'm missing something, but did he also over-speed the rotors?
Presumably he pulled in more power after the lift and didn't see (or hear) the RPM rise, but the clutch should have prevented the engine from going faster than the rotors so the engine can only over-speed as far as the rotors do. With the rotors, the damage problem initially is the bearings, not the blades.
He must have been low weight, otherwise I would have expected there to be a droop, not an over-speed, when he pulled in a lot of power, and that, in itself, would have caused more yaw.
I'd like to understand so I can try to avoid it happening to me!
Lafite