Without the system booted, what would execute the software?
I think Sten is referring to a sleeping or hibernating computer. . . The NIC card has a dumb processor on it capable of certain dedicated tasks, among which is sending a signal to the host computer saying "Hey! Sleeping Beauty! wake up! Someone wants to chat with you!" The NIC card is always listening for that incoming alert to the MAC address. . . unless it is powered down.