Ordinarily, a leaf shutter equipped camera with M synch delays opening of the shutter until a small amount of time after the flash bulb itself is triggered, so that light output can reach its maximum output (after the brief delay) while the shutter opens and closes. It is this delay time, and the design of the leaf shutter, which ordinarily permits M-synch to be across all speeds which the leaf shutter is capable.
In the case of th Minolta M16II, IIRC (I owned one about 5 decades ago!) the shutter is a moving blind which is more similar to a focal plane shutter, and the camera does not have an M-synch specific delay, so the shutter opening has to be long enough the the light output to build and the light is captured by the open shutter. Set a slow shutter speed and watch the shutter open and close, to verify my memory of its shutter action.