[Note: a sketch shows the flyweights/spring governor head connected through hydraulics to the fuel rack; a speed-setting (speeder) spring and a droop feedback linkage.]

Droop is the characteristic where the governor's set-point speed reduces slightly as the load increases, expressed as percentage: Droop = (no-load speed - full-load speed) / (rated speed) x 100.

Droop mechanism: the governor compares the engine speed (via a speed-sensing element - mechanical flyweights whose centrifugal force opposes a speeder spring, or an electrical speed pick-up) with the desired speed. In an electro-hydraulic governor the flyweight/speed sensor generates a "droop" signal. A droop feedback is produced by a linkage connected to the fuel rack/output which, on load increase, moves a spring adjusting the effective speed set-point downward by an amount proportional to the load (output). Thus, as the alternator load increases, more fuel is needed, and the governor slightly reduces the speed set-point. This gives a governor whose speed is proportional to load; several such alternators in parallel can share load stably.

Mechanism step: at no load the governor holds the engine at, say, 100% speed. As load is applied the fuel rack moves to increase fuel; the droop linkage (tied to the output/fuel rack) pulls against the speeder spring, raising the speed setting demand effectively so that the flyweights balance at a slightly lower speed. The engine stabilises at, e.g., 97% speed at full load. Altering the droop (adjusting the droop linkage/percentage droop knob) changes the slope of the speed-load line.

Isochronous governor: has zero speed droop; it maintains constant speed irrespective of load. The speeder spring force is not changed by load, so when load increases the governor opens the fuel rack to hold speed exactly at set-point. It is used where constant frequency/speed is essential, e.g. a single engine driving an alternator feeding a grid: the engine speed and hence the alternator frequency remain constant at every load.

Droop governor: the speed falls slightly with load (droop given as percentage). This is necessary for the stable parallel operation of two or more alternators (generating units) so that they share the load. Without droop, minor differences between two isochronous governors would make the machines "hunt" and fight each other, one taking all the load and the other overloading. With droop, if one machine tends to take more load its speed falls, it takes less, and load is shared proportionally among the machines set at the same droop.

Therefore: single machine + isochronous (constant speed); multiple parallel alternators + droop governors to share load. In practice the droop is commonly set (e.g. 3-4%) and adjustable.