If I remember there is an anchor pin/post at the top of the backing plate where the tops of both brakes shoes rest against.

With the brake adjuster between the rear and front shoe.

The front shoe contacts the brake drum which causes the front shoe to move with the drum.

The front shoe then pushes the adjuster in the same direction which then pushes the rear shoe in the same direction which causes the rear shoe to press against the anchor pin and up against the drum.

The energy from the primary (front) shoe caused the secondary (rear) shoe to press against the drum. Self energizing.

Hope my memory is not wrong.