Input: the orange roller installed on a plate that can turn around a fixed shaft. Center of the small roller must be above the line connecting center of the big roller and rocking center of the plate. The weight of the orange roller assembly causes the pressure at contact place between the rollers. The red arrow shows the recommended rotary direction due to which the pressure can be automatically increased in accordance with increasing load.
The orange spring is used to reduce the pressure in case the weight of the orange roller assembly is too large.