There is an exhaust port that would normally vent to atmosphere on the side. He drilled a hole on the bottom, welded a nut, and piped the exhaust down to blow the empty can out. It's a great idea and I will be adding this to mine next weekend. If you don't have a welder you could always use some JB weld to secure the nut to the bottom. You would need to buy some fittings and hose but only a couple bucks worth of supplies.
