The actuating arm is pivoted on the black round pin that is anchored to the brake shield.
The friction force is then caught by the stopper (Black trapezoid) mounted on the brake shield.
The force applied on the brake shoes is counteracted by one of the reactor springs (which one depends on the direction of the rotation) so that when the current through the electromagnet is withdrawn the spring ensures that the actuating arm is returned to its resting position and the brake shoes are retracted from the drum.
Kirchhoff's circuit laws are the basics for how to understand problems with the electrics.
Electrical faults involving brakes can range from annoying to dangerous so it is important to really get it working right.
When tracing faults it is important to try to see all symptoms, since that can lead to a component that is common for all parts.
If excessive braking occurs the electromagnet itself can suffer a burn-out through overheating and need to be replaced.