[Dovecot] How is the correct "auth" method selected in 1.0-stable?

In the example config for 1.0-stable, I see several "auth <blah> {}" blocks. The only one uncommented is the "auth default{}" block. How does dovecot decide which one to use? If I want it to use LDAP, should I just modify the "userdb" and "passdb" entries in the "auth default" block? Or should I be using the "auth ldap" block and somehow telling dovecot to use it?

Thanks!

-Ben