There is a cinch spring/barrel setup that acts as a one-way brake. It clinches when the jack is collapsing (either under the weight of the camper or with the crank/motor). Once cinched, the barrel inside the spring now has friction to the square plate which becomes a brake. The more load (i.e. camper weight), the more braking there is.
For this to work, there cannot be too much lube between the barrel and square plate or under the spring itself. Obviously the springs must be intact too.
Have you ever taken your jacks apart to lube them?
Refer to KKELLER14K's awesome post for pictures:
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/22881036.cfm
