They often take a running start and dive up out of the water and down to the bottom of the pond in a single fluid movement. They are itching and trying to find a tree to rub against. In most cases flashing indicates that they have some sort of parasite. The itching is only the beginning of the trouble parasites cause our fish. Fish often injure themselves by flashing against sharp rocks or other objects.
Some parasites pierce the skin of the fish and suck out blood.
Once the skin of the fish is broken, the fish become susceptible to opportunistic Aeromonas and Psuedomonas bacteria that enter through the wound or pinhole and cause havoc. Many times you'll think we're crazy for treating the fish for parasites when they obviously have an ulcer or other bacterial infection. The reason we do that is because the bacterial infection is most often a secondary condition caused by the parasite. You must get rid of the cause before you can get rid of the infection. Gill Flukes, for instance, are notorious for damaging the gills.