This swivel joint allows your second stage to sit in your mouth more comfortably by allowing the hose to move to a natural position.  Thus reducing stress and fatigue on your jaw, allowing you to more easily turn your head; it can also add comfort when sharing air with a buddy.

 The swivel features a 3/8x24 thread on both sides, so should suit most modern hoses/second stages.  It is made from chrome plated brass, with an internal steel ball it is corrosion resistant and supplied with spare o-rings.