You borrow 10 eth at 0.01 (0.1 btc total) and sell it. Eth crashes to 0.005, you buy that 10 eth back (0.05 total) and pay your loan. You end up with 0.05 btc minus expenses. If you used leverage, for example 3x, you earn 0.15.

You really need to know what you are doing, you can loose everything really fast,always have stop orders set to limit your losses. And do not pay too much for that loan.