In extremely low temperature operations, the rubber material in V-belts becomes stiff. At sufficiently low temperatures, the rubber will actually reach a glass point, where (as the name implies) the rubber is hard and will shatter like glass if it is bent.
This is an extreme condition but approaching this state the rubber will go through various degrees of stiffness. As long as a drive is in operation, the heat generated through drive friction and bending will increase the internal belt temperature and maintain belt flexibility in temperatures well below the normally accepted minimum operating temperature. Trying to immediately transmit a load through a belt which has taken a cold set can cause belt failure because the belt is too rigid to bend.