Why it exists
Saturation diving is a professional technique where divers spend days or weeks in a pressurised habitat breathing a helium-oxygen gas mixture (helium replaces nitrogen to prevent narcosis at depth). Inside the habitat, helium molecules, much smaller than oxygen or nitrogen, slowly seep through watch case gaskets and accumulate inside. When the diver decompresses (returns to surface pressure over hours/days), the helium can't escape fast enough; the internal pressure exceeds external pressure and the crystal can pop off violently.
How it works
The valve is a spring-loaded one-way release mounted in the case-side at 9 o'clock (Rolex Sea-Dweller, Omega Seamaster Diver 300M). When internal helium pressure exceeds external pressure by a calibrated threshold (typically 3-5 bar), the spring compresses and releases the helium. Auto valves (most modern) work without user action. Manual valves (Omega Ploprof, some Doxa) have a screw-down crown the diver opens during decompression. Either way, the helium escapes harmlessly and the crystal stays in place.
Who actually needs it
Almost no one. Saturation diving is professional commercial work (offshore oil rigs, deep marine construction, naval salvage). Recreational scuba divers max out around 40m depth and never breathe helium mixtures; their watches don't accumulate helium and don't need the valve. The valve is now a tool-watch credential rather than a functional requirement; modern dive watches with the valve mostly carry it as a heritage / authenticity signal rather than for actual saturation use.
When it was invented
Doxa and Rolex co-developed the helium escape valve in 1967 for the COMEX commercial diving programme; the Doxa Sub 300T Conquistador and the Rolex Sea-Dweller (1971) were the first wristwatches to commercialise it. Modern dive watches with HEV: Rolex Sea-Dweller 126600, Sea-Dweller Deepsea 126660, Omega Seamaster Diver 300M, Omega Seamaster Planet Ocean (auto valves); Omega Ploprof 1200M (manual valve). See wiki: helium escape valve.