Some of the ASW19s (and maybe the 20’s) have a fold down wood bulkhead. There is a 1/2 gap around the perimeter that lets the air escape into the fuselage where the air extractor is located. For gliders with a fiberglass bulkhead, the kit includes a template to cut some holes, and also some round plastic screens that prevent the anything other than air from getting through.

As far as glider performance goes, the difference is probably insignificant.. But when it comes to pilot comfort, the airflow through the cockpit was probably double what I had before, maybe more.