It is not all about rotor efficiency on the Sea King/ S61 , when conducting a flyaway with one engine in manual control (and therefore limited) and the 'good' engine pulled back, we decay the Nr to 91% to utilise the energy in the rotor to maximise acceleration - in the same way that the energy in the rotor is used to cushion an EOL.
Once above SSE speed, we lower the lever to recover the Nr to 100% and the aircraft climbs because the rotor is becoming more efficient.