That little "DISK" as you call it Bob,[please note my proper grammer usage,lol lol] is called a "RELUCTOR WHEEL" and used to measure wheel speed via the little sensor located right next to it.That sensor counts the little slots in the reluctor wheel as the tire/wheel assembly is turning,so if/when the wheel suddenly stops turning,due to a wheel lock up caused from wet roads,snow,ice,oil/gas on the road,or just plain hard braking in a panic situation,that sensor will send a message to the ABS control unit to unlock,or "PULSE" the brake pressure going to that caliper,thus preventing a skid,to hopefully PREVENT loss of control,and or bring the machine to a safe and controlled stop.
The Triumph Thunderbird I currently own,the SE model,comes standard with ABS,and I gotta tell ya,[even though I'm not a big fan of ABS] works pretty well as I had one incident where I would have either dumped the machine or made contact with the vehicle in front of me which caused the situation.But then again,I wouldn't stake my life on it either.Common sense,paying attention to surrounding conditions,and sensible speeds,will more often save ya from getting yourself into a bad situation.Dave!!!