Bike pump: what is this lump? What is this lump in the picture and how does it work?



I have another pump, which physically is the same but minus the bulge.  The one in the picture seems to perform better, ie less work to reach the same pressure.
This from an old article circa 1996 it references a Medai floor pump. "Screwed into the base near the compression tube is an air tight steel chamber. This chamber is known as a reservoir and holds a volume of the compressed air to assist in equalizing the pressure of the pumps compression chamber and the inner tube. This feature gives the tire a small assist in time as it "seats" on the rim."