C-Flex exhalation relief is a feature of some Respironics machines. C-FLEX is Respironics name for their method of exhalation relief.In a C-Flex machine the pressure is reduced momentarily when the machine senses that the user has started to exhale. In a technical disucssion Respiroonics shows that the pressure is reduced the moment the machine senses the user starts to exhale. The amount of reduction is controlled by the C-Flex setting on the machine. After a fixed time, the machine reverts to the set pressure.