Buying high pushes the price extremely high, allowing someone who can corner the market to profit by selling. Selling low pushes the price extremely low, allowing someone who can corner the market to profit by buying. If you're the one doing the high buying and low selling, you know when it's going to start and when it's going to stop, allowing you to time your trades until after the walls re-appear. Some lucky and/or wise counterparty recognizing this happening can both profit during the buy up or sell down (when you're buying high and selling low) by taking your trades and they can also profit during the buy low and sell high phases by taking the trades of the walls that other people put up.