With the help of the funnel shaped burner intake, the mirrored funnel of the flare at the burner’s tip helps to create the vacuum effect known as “venturi”.
The flare is made from a 3″ segment of 1″pipe that slides over the end of the 3/4″ burner tube. After heating the 3″ segment to a red heat, I hammer the rim of the pipe against a piece of round bar, rotating the pipe as I work. The flare should expand out to around 1 1/2″ at the opening. I recommend putting a set screw in flare as well, to provide a wider range of adjustments for your burner.