The service-brake chamber applies the brake by air pressure and releases it by spring pressure (just like a single service-brake chamber).
Using an opposite action, the spring-brake chamber applies the spring brake by spring pressure and releases it by air pressure. In the event of air-pressure loss (an emergency or an intentional exhausting of air by the driver, e.g., while parking), the power spring will push the diaphragm and push rod down and apply the brake. During normal operation, air pressure keeps the power spring compressed and allows the service brake to operate normally.
If air pressure cannot be restored and it is necessary to move the vehicle, the power spring can be compressed manually by the use of a wind-off bolt.