they place huge orders on the bid and offer. they pull their orders and put it on the other side.
eg. huge order to sell 2000 bund on the offer.
people sell because of the herd mentality.
the guy pulls the 2000 offer and puts 2000 on the bid.
the people who sold say "*****".
they screw with day traders and spreaders (cos they can flip 2 markets at once if they work as a team).
they can affect long term traders who use charts as they manipulate the market through levels - thereby triggering auto stops and stopping people out for their own profit.
it can be a very profitable strategy. the most well known is Paul Rotter, a czech who does 180,000 round turns a day on average allegedly, and makes (again allegedly) a few mill a week.