TTinCo-
Not quite true. The zombie programs can check back with the "master" for instructions.
Example (not really sure if this is how it's done, but way it might be done):
Hacker creates some hidden webpage. The program checks this page once or twice a day. The page contains IP addresses to attack and maybe a link to a new hidden page for tomorrow's attack.
So, the zombies can get updated instructions. There are much more sophisticated ways to accomplish communication with the zombies, but my example was just to illustrate that it is not difficult.