The auto-whitelist, or AWL, tracks scores for your regular correspondents in a small on-disk database.
The AWL is actually a very simple system. In short, the AWL is a score averaging system. It keeps track of the historical average of a sender, and pushes any subsequent mail towards that average.
So if someone that never sent you mail before sends you a mail that scores 20, and then sends you a second mail that would score 2.0 without the AWL, the AWL will push the score up to 11 on the second mail. This is auto blacklisting, based on their past history of spam.
If that same person sent you a mail that scored 0, and then later sent one that scored 7, the AWL would push the score down to 3.5. This is auto-whitelisting based on past history of nonspam.
But the "auto whitelist" isn't really a whitelist per-se.. It does however have a "learning white/blacklist" type behavior as a result of it's averaging.
Sometimes it may appear that the AWL is assigning scores the wrong way; read AwlWrongWay for details.