Yes, output is controlled by the duration of the flash.
In manual mode the duration is at maximum length.
Some advanced flashes offer in manual mode to choose out of several outputs (1/2, 1/4, 1/8 ....). Typically this is achieved by cutting off the flash after the resp. duration.
As indicated, your model is the most basic autoexposure one, More advanced models got several auto-apertures, several output settings in manual mode, swiveling reflector, communication with the camera.