It depends on what type of brakes you have also. From another source but it should cover most examples:
"If you have a hydraulic coupler (surge brakes), pulling the cable will pull a lever that depresses the piston in the brake cylinder, actuating the brakes. Depending on the type of coupler, there may be a ratchet mechanism that will hold the lever in order to keep the brakes applied so they will continue to slow the trailer even if it becomes totally disconnected from the truck. Some older couplers don?t have the ratchet so that after you release the pressure on the cable, the brakes release. Older Atlas couplers didn't have a ratchet. If you have a hydraulic coupler, either you didn't pull hard enough to engage the ratchet, the ratchet is broke, or you don't have a ratchet.
Examples of a hydraulic coupler include Tie-Down and Dico couplers. They do not require an electric brake controller to be mounted inside the truck and the brakes will work regardless of the vehicle pulling the trailer.
If you have an electric/hydraulic system, pulling the cable will activate a breakaway controller which will send an electrical signal, via a battery, to the actuator telling it to apply the brakes. Examples of electric/hydraulic controllers include such brands as Dexter and Brake-Rite. These types of actuators require a brake controller to mounted in the vehicle and all vehicles pulling the trailer must have a controller in order for the brakes to function."