In normal operation there would always be a substantial load on the tow cable, so my guess is that the brake is triggered if the load falls below a specified level. It's probably something like the slack line detector/alarm/shutoff systems on cranes, which stop the hoist before the wire rope can wander out of the grooves in the drum. I could tell an interesting story about my experience with one of those, but this is probably not the place.
LittleInch, yes, I did retire in 1992 at age 46. I had several years warning that my job would go away if I didn't want to relocate, so I started saving and investing everything I could. I got a substantial bonus payment for volunteering to leave a little before I'd expected to. My mother, who grew up during the Great Depression, broke down in tears when I told her I no longer had a job.
My plan (if you can call it a plan) was to start looking for a job if my nest egg ever fell below half of what I started with. That never happened, so I spent my time sailing, building and flying model planes, learning how to operate machine tools, getting my private pilot's license, owning my own plane, and doing welding, machining, and fabrication at the local tech college. I started my latest hobby of tree climbing a couple years ago. I'm a huge fan of early retirement, and I marvel at the fact that I was able to pull it off.
