Put simply, the additional engine power is used to maintain the rotor RPM at higher pitch settings than previously would have been allowed, so more rotor thrust can be maintained and thus higher MTOW or hover ceilings are created. There have also been revisions to the main rotor blade profiles over time (the Squirrel AS350 / AS355 being an example), but rarely is the rotor speed is increased to generate more thrust (to the best of my limited knowledge).
Last edited by ApolloHeli; 26th June 2021 at 19:36.