Flash orders are small orders that "ping" the market continuously. They have a life span of milliseconds and are not placed to buy or sell stock. Flash orders have one aim. To determine order flow. Its not unlike Jesse Livermore's strategy of tossing out a couple market orders to discern how the market takes them. However, these orders are rarely filled. On high liquid stocks....they are constant. They last literally milliseconds before they are cancelled. These algorithm's sole design is to find out what you want to buy, the price, and buy it before you because they have the competitive advantage of robust servers that are located within the exchange and sell it to you a tick or two higher. For this, the exchanges pay them a rebate for "providing liquidity". They get paid up to 1/3 of a penny by exchanges. So, on a $20 stock. By buying and selling at $20 - they make 2/3 of 1 cent. Doesn't sound like much. Well, as I said, these orders are constant on high volume liquid stocks. And when you add that to the tick or two profit they make on a trade......It can quickly add up.