zac
in a helicopter the engine is governed so the power the engine makes is the power required to turn the rotor system (MR and TR) at the same RPM, if the lever isn't moved then normally less power is required to turn the rotor and the governing system would reduce the power output of the engine(s) accordingly.
finer points that vary this may be if the helicopter had been hovering at a very good Lift Drag ratio and it gets much worse but that's the other way around normally OR some helicopters run different RPM often commanded by a TR pedal position sensor, if it commanded an RPM rise then more power may be required.
Neither of these sounds like what is happening here. It sounds like this helicopter is broken (more pitch being added due flexure of something), (or has a pusher prop with varying pitch !)
Pls OP tell us more . What type?