The Sony camera, A-6XXX APS-C series and the new full frame version as well, always fire a pre-flash. Even if the camera is set to Manual mode. There is some good and some bad there. Using Inon S2000, D2000, Z240, D200 and Z330 strobes you can accomplish sTTL with the camera in Manual mode. You can set aperture and shutter (up to 1/160) and with the strobes in sTTL and using the flash exposure compensation of the A-6XXX under the Function button to control exposure and fine tune with the knob on the strobe +/-. Or you can set the camera flash exposure compensation to 0 and just use the sTTL control on the Inon strobe. I find scenario one more flexible for my uses. A typical starting settings for me with Inon D2000 strobes is f5.6, shutter at 1/125 and camera flash compensation at 0. Since the strobes output occurs in a tiny slice of time, like 1/2000 second, the shutter speed has little effect (well, you know, some yeah, especially ambient and background) but the f stop has a large impact on the exposure. By the way, in the sTTL strobe setting the magnet switch has no effect as the pre-flash is default for the Inon strobe and obviously needed for the sTTL to function.
You can quickly set to strobe manual exposure, magnet out on the S2000, D2000, D200, selector set for pre-flash on the Z240 and Z330. Set camera in manual exposure, set, under the Function button, flash power to -3 stops (to conserve battery) and as starting point, again1/125 shutter, f5.6 or f8, and match set the strobe to the same f stop on the dial. Adjust camera aperture (f stop) and f stop settings on the strobe to get your exposure where you want it.
James