ABG
ABG seems to come in several forms. The older sensors often pulsed the vertical gates during integration to cause overflow into the chip substrate, rather than down the pixel column. This method was possible with the old Texas TC244 etc. and didn't lose any area, but gave rotten linearity.
The Kodak sensors are generally made with anti-blooming structures on the chip surface, which act as overflow drains and draw off charge before it gets excessive. This works well, but it uses up sensor area and reduces the effective QE. It is also 'permanent'.
The Sony chips have variable anti-blooming, which is applied through the back of the pixel by varying its offset voltage with respect to the CCD substrate. At some point, dependent on the bias voltage, the pixel begins to 'zener' into the substrate and charge leaks away. This kind of AB gives the best of both worlds. It doesn't suffer from severe non-linearity until you are very close to 'full well', it is adjustable over a wide range - including 'off', and it does not use any surface area from the sensor.
All of the Starlight Xpress cameras have 'vertical anti-blooming' as in the MX916. Because it is hardware controlled, all of the available software supports it. The AB level is controlled by an internal potentiometer, as the linearity with AB 'ON' is so good that we never imagined that users would want to turn it off! :-)
ABG & HX916
The HX916 is an ABG chip, but is different from the Kodak and other ABG versions, in that the anti-blooming drains are underneath the pixels and do not affect the sensitive area. Other aspects are that the ABG does not affect the linearity to any significant degree up to around 80% of saturation (so turning it down is unlikely to give any useful sensitivity boost -- any increase in gain will be slight) and you can turn it off by adjusting an internal control in the camera.
It is quite easy to reset if you wish and is accessed by removing the rear disk and sliding off the camera barrel. On the main PCB you will find two small potentiometers, one on each side of the board. The one that you should adjust is on the same side as the big 15-way connector. Turn it clockwise to reduce the AB level (turning it anti-clockwise by about 15 degrees is enough to switch off the AB). Too little AB will result in strong 'smearing' of bright images, too much will give grey 'flat' highlights.