man... they look dangerous. Presumably they stop the pressure getting to high with the springs and rollers.
I put a pair of Suntour Self Energising cantis on one of my GT's... on the front. I hadn't really considered how they worked so I put them on the front. The manual said 'Do not install the brake on the front as they are too powerful and injury may result'. Of course they were rubbish as the mechanism worked the opposite way round.