GC, I know only what I read but it makes sense to me. The point is that the spring rate is such that gravity on slopes up to some pitch (unstated) and normal slowing down without the use of brakes does not compress the spring and activate the trailer brake; whereas the (greater) g developed by applying the car brakes does compress the spring and activate the trailer brakes.. Have you read the full article Jim referred to?

Regards – Chris N