You have a receptor in your airway that constantly monitors CO2 levels. If these CO2 levels are elevated for some reason, for example, in an enclosed space, and you have a central nervous system hypersensitivity to CO2, then it can trigger a panic response. Other people have a normal tolerance for high CO2 levels so will not react in the way that you do.
When you exhale into bcalm the air is diverted out into the environment. In its place, you breathe in air that has been filtered by the device to reduce CO2 levels to that of forest air. In effect, your air supply has been 'scrubbed' of high C02. As such, your airway CO2 sensors stop sending messages to your brain saying that you need to escape.
This CO2 'scrubbing' process is very safe, and is also used in anesthesia machines and in rebreathers used by divers. After about 6 or 7 breaths, the CO2 levels in your throat are back to a much lower level. This means your CO2 receptor now sends a message to your brain, saying "relax, it's okay". It feels as though you just stepped out into a forest, but you haven't needed to go outside.