"What Narang and other high frequency traders tell their computers to do is to make a profit of a penny or less, 40 million times day."
It artificially changes the prices that others end up paying for the stocks. Every time you or I buy a stock or do a trade whatever we are buying has had the price changed by these trades.
Shit like this is why Wall Street needs to be regulated so people do not manipulate it just for personal gain.