If you are looking to understand the concept, here is something that i wrote http://www.directorytrials.com/link-bid-directories-concept-explained/
Bid directory - same as any other directory however you bid for a position in the category you are wishing to have you website placed. The highest bid simply climbs to the top of the category. If the highest bid is at $70 in a travel category and there 8 pages of this category with the lowest bid being $3 and you place a bid of $35 then you will climb above all sites that paid less on their bid. Your website will always remain listed but as someone bids more than you your site will fall lower in pecking order of listed sites. Hope this helps. .