^^^^ Clodmaxx has it right, not sure why ebay structured it that way, but I've used BIN after there were other bids on an item. Not sure that made me popular with the previous bidders, but it was a sure hit with the seller.

I guess in some cases it lets the seller get his reserve when otherwise he might not have been able to through bidding. Not every auction sets off a bidding frenzy as some seem to do.