First YoDaddy sends them to auction (10 days since they initiate the auction, when customers do they only get 7 days) if the owner doesn't renew in the grace period (17 days) If no one bids on the domain they give it to the 1st backorder as to their priority (which still remains a mystery, many who should be
#1 to get the domain simply don't get it) If there are no bidders AND no backorders exist it enters the Pending Delete stage where domains are given to SnapNames. If no one picks up the name there a while goes by waiting for the 42-day total redemption period to be used by the registrant, after that if the owner doesn't reclaim it's available for public reg 7-10 days later (this period changes depending on the registrar)
Note that other registrars send unrenewd domains to SnapNames, NameJet or Pool sooner, you have to check this. But with MoDaddy domains don't show up on the dropped domains lists on SnapNames until they reach the Pending Delete period
:-/