Here is a pretty thorough paper by someone at Woodward :
http://www.woodward.com/pdf/ic/26260.pdf
Most/all of the helicopter engines prior to recent FADEC* equipped engines operated with droop type governors, and the later ones had a droop line reset device, known by various names, such as collective bias etc.
* GE did implement an analog electronic control ( named PMS, for Power Management System ) on the T-58-5 for the SH-3D, which provided isochronous governing, and it worked well......when it worked. Unfortunately there were enough instances of failure at the wrong times so that a PMS-OFF switch was added to the SH-3D cyclic grip.
Thanks,
John Dixson