The actuating force for the wheel brake is generated during braking of the towing vehicle, as the trailer tends to overrun and
pushes the drawtube in. This force is transmitted to the expander devices in the wheel brakes via the overrun lever, brake rod,
compensator and brake cables.
As a result, the brake shoes are pressed against the brake drum. The resulting friction from contact of brake linings and drum
produces the required braking.
On releasing the brake on the towing vehicle, the drawtube returns to its original position and the brake shoes are released from
the drums by means of the tension springs.Auto-Reverse
During reversing the drawtube is pushed in and the brake shoes press against the drum (as in forward braking).
When the trailer wheels are in reverse, the reverse rotation, combined with the friction generated from drum and lining contact,
causes the sliding shoe to slide on its carrier towards the adjuster.
This displaces the sliding shoe radially inwards away from the drum. The drawtube may then travel fully to its end stop without
applying further force to the brake shoes and the trailer can be reversed without any difficulties.
Changing direction from reverse to forward travel results in the drawtube being pulled out, allowing the sliding shoes to be
returned to the forward braking position by means of the tension springs.
From this guide
which contains more information also.