Registered members at GreatCollections can place a confidential maximum bid on any auction. If there is other bidding activity, GreatCollections will bid on your behalf up until your maximum bid.
For example: John Smith would like to bid $1,000 on a 1881-S Morgan Dollar, even though the minimum bid is only $650. John enters his bid and if there are no other bidders, he will win the coin for $650... the minimum bid.
However, if Peter Foster places a bid on the same coin at $800, GreatCollections will bid on John's behalf and John will win it for $825 (one bid above the second bidder).