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.

For example, say that Bitcoins were trading at $9 or so and are stable there. And then say that last Thursday night, someone wanted to push the price down to $7 so they can buy low. In order to do that, they have to sell coins. So first they sell at $9. But then they have to keep selling all the way down to $7. So they have to sell below market price. Since the price will go back to $9 when they're done, a counter-party can profit. Now, while that someone is buying at $7.80, others can buy at $7.80 too. Since the price will go back to $9 when they're done, again a counter-party can profit. If people with sufficient resources recognize this manipulation and move on it, the person doing the manipulating will lose because they'll have to sell shares below market price and they will hardly get to buy any shares back below market price. Heck, they might not even be able to get all the way to $7, getting stuck just under $8. It could have happened .. last Thursday night.