The flickering you see during start up and shut down is the load being switched from engine to APU generators and vise-a-versa. Power is usually switched at the gate to ground power so the APU can also be shut down.
The engine generators will automatically drop off line as they slow down and the output falls out of spec. The arrangement on most aircraft has the AC power switched to the next available source automatically.
Windmilling engines can supply hydraulic and electrical needs depending on airspeed and type.