One simple technique:
ShillA bids a ridiculously high bid. That advances the auction to one
increment past your highest. That ridiculously high bid is withdrawn.
ShillB, now knowing your high bid, bids one increment or less below your
high bid.
You "win" at your high bid.
ShillA and ShillB could be the same person. If the current bids are in
the $200s, if the ridiculously high bid has an extra zero at the end, eBay
is likley to believe that $2400 was a MISTAKE, and was intended to be
$240.
The other common technique:
ShillA simply bids competitive bids. If ShillA accidentally wins, then
the bid is withdrawn, as a "mistake", and the seller offers you the item
"second chance" for your now-revealed high bid, instead of the highest
that it had gotten to before ShillA's "mistake".
