I've written about it briefly before on HN, and it's very beneficial but not proprietary tech. Basically we create a "shim" database containing our app's sprocs, views and functions, and additionally views to the transactional database tables in another database.

That means that release N can be pointed at the "real" database, while release N+1 can be pointed at the shim DB and they're both using the same transactional data. You can run both in parallel until you commit to N+1, whereupon you shutdown all N app servers, update the sprocs, views and functions in the main DB, and (optionally) point your web servers back at the main DB. There are a few other details to take care of, but that's the gist of it, and the details are minor, or at least we found them to be.

Disclaimer: the above is my experience on MS-SQL at a pretty good sized eCommerce site. Other RDBMSs may not work quite as well, or your app may use heavily features that don't work in "shim mode".

Ah, very cool, thanks.