Drum brakes have a natural "self-applying" characteristic, better known as "self-energizing."
[5] The rotation of the drum can drag either one or both of the shoes into the friction surface, causing the brakes to bite harder, which increases the force holding them together. This increases the stopping power without any additional effort being expended by the driver, but it does make it harder for the driver to modulate the brake's sensitivity. It also makes the brake more sensitive to
brake fade, as a decrease in brake friction also reduces the amount of brake assist.
So, if going in reverse, it unapplies itself.
