Manual:$wgAllDBsAreLocalhost/en

Details
Make all database connections secretly go to localhost. Fool the load balancer thinking there is an arbitrarily large cluster of servers to connect to. Useful for debugging.

For example you might have a configuration like:

Which would cause MediaWiki to behave like it had 6 db servers (1 master + 5 slaves), when really you just have the 1 DB on localhost.