If you see that your coin falls in price and the fall will continue further, then it makes sense to sell it and then buy at a cheaper price. Thus, you will have on your hands the same amount of coins, as well as profit from the difference in the price of sale and purchase.
Your advice sounds easy at the first view but is very difficult to do. How are you sure that the price will drop further? It's also possible that the price will go up again just after you sold. Do you have a strategy to know if the price will keep decreasing?