When inhaling, the diaphragm assembly (item 1) is drawn in towards the lever arm (item 2) which lifts the sealing tip (item 3) from the valve seat in the connection nipple (item 4). The breathing gas can now flow into the valve. When inhalation ceases, the diaphragm assembly, the lever and sealing cone all return to their starting positions, the inlet closes under the influence of the sealing spring (item 5).