From this amateur NTSB sleuth: I don't know anything about R-44's, but my guess would be that the last tail rotor drive shaft coupling (the one that attaches it to the gearbox) came undone. That left the shaft flopping around back there and it sawed the back end off. I've had *two* tail rotor failures in 206's. In both cases couplings came apart. Each time, the unsupported shaft sliced a big hole in what was below it - in this case of the 206, the tailboom. Seems to me that the R-44 t/r driveshaft runs inside the boom?