Once it is mature, it erupts from the cyst and attaches itself to something fixed, (rock, plant etc.), where it builds a strong skin around itself. This stage is called a "tomont". This is the principle replication phase. Inside it's protective sheath, it divides and divides and divides. Eventually, the skin ruptures and many of the new, free swimming stage tomites start swimming around trying to find a fish. There can be 1000's from a single cyst.
Note, the cyst on the fish, and the encapsulated tomont are pretty much indestructible. It is only the free swimming stage that is killed off by medications. The reason the heater is often screwed up a little when treating Ich is not to kill the parasites, but to speed up the life cycle and get them to the free swimming stage faster.