when you fill your gun direct,,, it has a check valve in the gun
well it is the same principle,, it holds the air in the tank when the manual valve is open on the tank
benefit on starting the compressor, open tank, start compressor, check holds tank pressure in the tank
benefit on shutting down the operation, quickly open bleed valve on compressor, let compressor run to cool down,,, close tank vlv and bleed hose by tank bleed vlv
first thing I bought when I started filling my own bottle,, number 2 was an ez fill