After closely looking at it, I think I understand how it works. The adjuster turns the screw, to make the assembly the correct length. Then the cable part moves the whole assembly when the lever is applied. The concave portion on the round metal enables it to be engaged, or disengaged (like Star Trek !). When disengaged, it does not look like there is anything to move the assembly back away from the brake pad, but maybe that does not matter since there is no pressure being applied anymore.
The screw could rust inside the piston, making it unadjustable. The piston could also rust inside the caliper. These 2 things must cause most of the brake problems.