Isn't it to do with the "keel valves" as discussed below:
Yes.
Also discussed in 2007 by Aztec Kid
http://www.pprune.org/tech-log/29124...ise-again.html
& a Boeing Patented design 1983:
Patent US4463774 - Fuselage-mounted valve for condensate drainage and cabin-air pressurization - Google Patents
"The invention pertains to condensate draining, air pressure sealing valves that are mounted on an aircraft fuselage at various positions along the belly to drain off condensate collected in the bilge-like area of the fuselage, and to close in response to cabin air pressurization to form an airtight seal."
On A320 they open / close around 1.8 delta psi.