In a live tax lien auction a bidder will lower his bid in one percent increments until everyone else leaves the bid or the investor's interest goes below an acceptable minimum level. In order to determine this, the investor must be clear about how low he/she is willing to go on specific properties. At that point the investor will drop out.
Proxy bidding is a form of competitive sale in which bidders enter the minimum interest rate they're willing to accept for each certificate. In Maricopa County Arizona the auction system acts as an electronic agent, submitting bids on behalf of each bidder.