It is a ”brake/force modulator”.å
It ”works” under the assumption that people try to control their braking by the amount of lever movement. So for the amount of wire pull corresponding to the compressibility of the spring, the brake’s pinch force is determined by the characteristics of the spring rather than the amount of lever movement/cable pull.
Done ”right”, it means that pinch force increases a little slower WRT lever travel than If the brake is set up without brake modulator.