That's a surge brake box. As you slow down the weight of the trailer creeps up on the towing vehicle applying hydraulic pressure to the brakes. The cable is there to activate the hydraulic brakes in case of a break away. You can see the oil fill plug in the top and the heavy duty spring behind it that releases the brakes.
This system is used almost exclusively on boat trailers since dumping electric brakes into the water can cause issues.
The real bitch with these is trying to back your trailer up an incline. Wants to keep applying the trailer brakes.