The key to make a flurry of DM's is that you need to have a good amount of speed, and a good target area where you are hitting. If you want to get a boom hit you should go for somewhere on the body where there are many joints close together, and try to always have a follow up attack to be more fluid if you are doing a madman. Usually to get DM's you would use your Feet, Hands, and sometimes even your shin to get the DM but you will always need speed for a smooth, and clean hit. But the guys you see on youtube have been practicing for a long time so you shouldn't expect to be like Larfen in a month so just practice and in no time you'll be dishing out all kinds of DM's.