It's been a while since i installed it, but basically, it's got a standard power lock actuator mounted to a bracket, which fits between the existing lock assembly and the inside of the tailgate. Also attached to the bracket is a metal plate (i think everything's aluminum, but i could be wrong) that is moved back and forth by the actuator.
Now, if you look at the inside of your tailgate latch, the external handle is attached to an angled steel bracket that folds outwards to disengage the locks. When the actuator goes to "lock" mode, it slides a metal plate underneath that bracket to jam it. Since the internal portion of the handle is angled, until i notched the handle, i was able to force it to slide over the metal plate. Now the notch catches the plate. The plate is relatively thin (maybe 1/16"?), but 1/2" tall, and is well-secured in a pair of hard plastic bushings, which are mounted on the backing plate, which is of course mounted to the tailgate and latch assembly.
Now, this isn't as good a solution as your ordinary car door lock assembly that disengages the handle from the lock mechanism, but it holds up pretty well. i've tried to force it, to the point where i backed off out of fear that i'd break the plastic handle, and it did not give. However, by doing so, something in the assembly, possibly the hinge, dimpled the paint above the lock. i don't doubt that the whole mess could be ripped out in about twenty seconds with a crowbar, but it's enough to deter the casual thief, which is really the most you can hope for with most vehicle security.
Cheers,
...dave