Matching Orders
Entering identical buy and sell orders at the same time to create the appearance of active trading in that security.
Investopedia Commentary
Many investors will buy a stock if the volume increases because it is interpreted that news is about to come out. Matching orders costs virtually nothing for brokers. This is a highly illegal practice.
Related Links
The Basics Of Order Entry
The Short And Distort - Stock Manipulation In A Bear Market
Investment Scams Tutorial
See also: Buy, Manipulation, Painting the Tape, Sell, Volume