auditing, by definition, is manually counting some of the votes by hand, and checking them against the machine counts. the samples are chosen at random.
currently there is no auditing whatsoever. the HR550 bill proposes to randmonly select 2% of the precincts, including at least one per county. the entire precinct is selected by random and hand counted. if there are discrepancies outside an acceptable level, a larger hand count would be initiated.
most people I talk to are unhappy with the way the bill is worded for many reasons and one of them is that 2% is not a large enough sample to reasonably ensure that fraud is not taking place. and there are also some people who do not want to support anything other than 100% hand counted paper ballots, but that discussion takes place elsewhere.
Rush Holt, the author of the bill, has responded to this concern by inviting people to push for an ammendment to the bill favoring a larger sample.
you can read the text of the bill here:
http://www.govtrack.us/congress/bill.xpd?bill=h109-550I am definitely not an expert on it but there are many discussions on this board that you can find many opinions on the bill, by searching for HR550.
The sad thing about it is, it has little chance of even going for a vote in the house unless the Dems take over. The repubs have kept this locked in committee for several years.