Qavion is correct:
variable speed input drives a hydraulic pump. This in turn drives a hydraulic motor which drives the generator. The intention is to produce a constant speed from the motor. The stroke of the pistons in the pump, is varied by the angle of a swash plate as they go round and round producing a constant output. Long stroke for low input speeds and vice versa.
Also present; governor, control piston (moves swash plate), oil lubrication, oil cooling jets, scavenge pump, fuel cooled oil cooler, filter, deaerator and of course the electrical generator itself rotating at 24000 rpm when online.
The IDG disconnect is a solenoid which drives a pin into a spiral track somewhere on the input shaft. A couple of turns and the gearing disengages itself.