To distribute the total torque requirements of the main transmission the NR has to change with increasing Tq. The S61 'droops' thus providing the necessary change of FCU datum. Moving the SSL's forward shifts the system datum thus compensating for 'droop' and at the same time increasing the rotor inertia - a good thing during take off and landing Cat A.
The SK's RR Gnomes use computer controlled FCU with electronic 'anticipators' connected to the collective so that as power is increased via the collective the computer datum is also raised. Torque sharing is again based on the need for a moving datum*, one that changes with collective position but this time it happens to go up rather than down as with the old CT58 hydro mechanical system.
*With the technology available to the S61 engine designers at the time (1950's) the problem of equalisation of engine output was one solved by scheduling each engine's torque output in line with a descending Nr. If you try it with a constant Nr datum then neither engine will be able to decide whether to add power or reduce power.
That's as best I can recall from the SK course in January 1970. :-)
G.