The American Horticultural Society Pruning and Training : A Fully Illustrated...

When placing your bid, try bidding the highest mount you're willing to pay (Maximum Bid). We will then continue to automatically bid for you while you're away, only bidding just enough to keep you in the lead. Your Maximum Bid is hidden from everyone else.

For Example:

• If the current maximum bid is $30 and you make a $50 Maximum Bid, we will bid $31 for you.
• If there are no other bids and you win, you'll pay only $31.
• If another buyer bids $31, we'll continue to automatically increase your bid until your maximum bid of $50 is exceeded.