Cryptocurrency market has a volatile nature and that's why nobody knows when market can falling and when it will bounce back. I think when market is falling, there is a big chance to buy a Bitcoin or a good potential coins at very low price. And hold them till the market will recovers and whenever your buying coins will come back to their high price, then you sell them and definitely you will get good return.
It's good in theory, but in practice you may face with lots of insoluble problems which will lead you to losses. Moreover, periods of rises and falls may be different. We should take it into account when creating a strategy!
It seems to me that the only strategy of earning on the fall of the market is the opportunity to earn on the difference in price, When there is fluctuation in the value of the crypto currency. But it will be a huge risk, because no one can guess what will happen next minute.