If you got luck when the price goes down a little then you buy and when the price goes up a little you also sell but thats a little risky and need a huge capital for that short trade mostly people doing that if they like a day trading.
Not luck but experienced and knowledge, those traders who play short trading are really risking their money, they've got a good idea basing with their knowledge and experienced, it's really possible to earned even in a down market if you are monitoring and studying the flow, what is important before doing such actions inside this industry, you need to have lots of patience finding moving coins that have a lots of players and understand well how the
people play around to take some advantage and earned.