This article is from the eBay FAQ, by William B. Shaw email@example.com with numerous contributions by others.
Bid shielding is done by a buyer to keep the price high until the end of an
auction to discourage other bidders. The high bid is then retracted leaving
the buyer's second account as the new high bidder. Note that a buyer has to
have two separate accounts or a partner to accomplish this. With eBay's new
rules on late bid retractions, it is more difficult, but not impossible to
shield low bids.
If you suspect bid shielding, but aren't sure, post the auction link to amoe
and let the group help.