Consider the following examples:
Example 1:
A broker knows that a large institutional client is about to place an order to purchase a large quantity of stock. The broker buys the stock first at the current market price, anticipating that the price will rise once the client’s order is executed. After the order is placed, the stock price rises, and the broker sells the stock for a profit.
Example 2:
A trader at an investment bank sees that a client’s order to sell a large amount of stock will likely cause the price to drop. The trader sells the stock in advance, then buys it back once the price has fallen after the client’s sale, profiting from the price difference.