In a Ruston gas turbine, why are there two Halon cylinders in each bank? How do they function?

The first bottle is the ‘first shot’ and the second bottle is ‘Extended shot’. The first bottle discharges the Galon into the hood through a 1’’ pipe in approximately 15 seconds, where as the extended bottle discharges the Halon through a ¼’’ pipe for another ½’’hour period to maintain the inert atmosphere.