The same way you'd earn money shorting a stock. If you "know" the value of something is going to drop - you short it.

Here's a hypothetical:

1. You borrow 5 ETH from your exchange.
2. You IMMEDIATELY sell that for $8. (So you currently have $8 in your pocket. Keep in mind you need to return 5 ETH to your exchange. )
3. You wait for the price of that ETH to drop, you purchase it back - at the lower price, of $4.
4. You return that 5 ETH to your exchange, and in the process have made $4.