Red Frog - Vic replied:

leave it open when travelling so it greats pressure inside to keep out dust.

I've never heard it called mega draw feedback though.

Member - Cameron B (WA) replied:

G'day Andrew, I worked for large caravan manufacturer for a number of years and can assure you the purpose of the "pressure hatch" fitted to your roof is to pressurize the internal airspace of your van whilst travelling. This basically keeps dust out by creating a positive air pressure inside your van when you are moving. Keep it closed during dusty driving and any vents or air gaps will create a "vacuum" or negative air pressure inside the van and you will have loads of dust to clean up afterwards (or the missus!!). Hope this helps.