Well to make it quick the answer is YOU DON'T. You cannot profit from something when it is falling down, but it is when it is recovering is the time you will profit. That is why a lot of people short (sell) their holdings when they know it is time to jump and stay in fiat currency, they will then buy their shares/holdings back when it has settled in or is recovering, with this method they have increased their holdings as they bought their cryptocurrencies at a cheaper price resulting into a profit when they sell at a higher price.