ON the H300 (& possibly other makes) there are shear pins/bolts afixing the mast to the tranny output. If the tranny sizes, pins/bolts shear and there is still (hopefully) a certain amount of rotation by the main blades.
Thinking about it though, the tail rotor would stop with the tranny. So there's a failed TR situation to contend with at the bottom of the auto.
Cheers & fly Safe. .Hone
Ps: The shear pins/bolts were explained by a nice LAME after the same idea struck me (hope he wasn't doing the lame, lets have this pilot on thing)