I was told they actually want you to roll the throttle down to idle in this situation. You’ll still have enough controllability and potentially save the engine from a giant over speed
You won't save the engine from an overspeed, it will have done whatever overspeeding it was going to do at the instant the shaft snapped. From the military Kiowa manual, the civil manual, at least the copy I have, is rather perfunctory in detail.
Main Drive Shaft FailureThe following symptoms occur
a. N2 RPM rapidly increases while rotor RPM decreases
b. Yaw left
c. Engine noise increases
d. Low RPM audio activates
e. N2 stabilises 103% or above
Recovery
1. Collective – Adjust to maintain rotor RPM
2. Confirm failure
3. Throttle – Maintain fully open to provide tail rotor thrust
4. Select landing area
5. Call mayday
6. Transponder – EMER
7. Brief passengers
8. Harness – Locked
9. Visor – Down
10. Land – Power on autorotation
Note: Because tail rotor will maintain constant RPM throughout the autorotative landing, tail rotor effectiveness will be greater than usual. Care should be exercised in compensating for yaw on touchdown.