Their brokerage is only one per stock per day no matter how many trades. They pay $100000 to Stock Exchange for this privilege.The aim is to buy or sell a large share block at a particular price. The computer program works it all out with price/volume being the main input. HFT [high frequency trading] is the technique of finding out what the real market depth is by 'pinging' the Exchange and looking at other peoples buy/sells orders. This trading technique was perfected by Goldman Sachs enabling them to skim a margin off many markets. It will eventually bring markets down to being inoperable.
