If you push that pin in,it will release air,to fill a rifle the adapter screws on the reg. and then you turn the thumb nob and that pushes down on reg pin and air will come out of the hose and into the gun tank . To fill it ,it has a foster fitting on side of reg. for filling . I do own the 13 cu,in and jda reg,mine has a 2200 out put,just for my maurader setup.I use this setup ,just for hunting ,i get about 4 2200 refills ,thats plenty. Then when i get home ,i have several scuba tanks from 60,80,100,at 3420psi.Hope this helps you out ,Paul