A solenoid is sometimes used in a circuit that senses current demand, and uses a solenoid to pull the throttle on until the current demand is gone, then it goes back to idle.
Auto Throttle senses load. When no power is required, this feature allows the generator to idle, saving fuel, and allows for quieter operation. When power is required, the generator will sense the load and provide full power as required.
Some engines also use an air vane to control the throttle.
As far as a log splitter, I would think you would want to run it at high rpm all the time.
On some generators, there is a manual/auto mode. If you are using the generator for lights and computers, etc, you want it in high rpm. If using drills or electric pump, the auto mode will cause the engine to idle until the drill or pump calls for more voltage/amps.
