The reason for drooping the Nr on the S61/Sea King is to utilise the extra energy in the rotor system to help achieve a safe single engine safety speed - once this is passed, the Nr is allowed to rise again (usually giving a climb because 100% is where the rotor is more efficient.
We utilise the same technique for both flyaways (from the hover or forward flight) and for single engine water take-offs.
Unlike some smaller singles, the Nr is not more efficient at 91% and that is why we don't droop it in auto to gain extra range - we use 104% for range auto.
The Nr rises with application of power in the SK but has to be manually set in the S61 - the reasons are to maintain the rotor efficiency and prevent static droop taking Nr below 100% since it gives you more NR to play with following an engine failure and prevents coning (if the rotor slows) reducing the effective size of your disc.