Cap seals over the first-stage air inlet when you snug down the yoke screw of the regulator in place to protect your regulator from having water enter inside the delicate internal mechanism when washing your gear after diving. Impact resistant non-corrosive cap also protects humidity from entering your regulator when stowed for long periods of time. Cap comes with a loop of line to attach to regulator yoke, so its right there when you need it.
