Action of electric brake inside the drum
For the technical guru, notice that the oval shaped object at the end of the curved lever is an electro magnet. Notice also that as it moves to the right while expanding the brake shoes, it is increasing its distance from the rotation center of the drum. Not shown is a ring of metal inside the brake drum with a radius just outside the position of the magnet. When the magnet is energized during a brake application, it draws itself outwards towards this ring of metal, increasing said distance from the center of rotation, and at the same time causing the swinging action which expands the shoes against the drum. QED. Your science master will further help you work this out if it is still not clear.