Hi Keen
Did you notice if there are any wires going to the brake? I can not see any in your pictures but it is possible they are coming up from inside the Stepper Motor. This brake could just add resistance to the Ball Screw which when added to the cogging of the Stepper Motor would prevent the Spindle Housing from dropping. Or more likely it is a electrically activated brake that is released when the Z Axis is moving and locked when it is not. Take a look at the Web Site below and you will see some similar brakes. I notice these units have a lever sticking out from the top that is a manual release lever. If you look carefully at Keens pictures you will notice a similar lever sticking out from the top of the brake.
http://www.electroid.com/prod07.htm
Willy