A hard reboot is where the power goes all the way off, a soft reboot is where it doesn't. A fork bomb makes it very hard / impossible to trigger a soft reboot, forcing you to do a hard reboot.
As an extra sting, a hard reboot can be damaging if the software and hardware is not correctly handling power interruption, which was much more likely in the 90's.
To clarify a little - a hard reboot in this case is not performed by issuing a shutdown power off command, which is safe, but by pulling the plug from the wall (the worst shutdown) or holding the switch down (quite bad).
As an extra sting, a hard reboot can be damaging if the software and hardware is not correctly handling power interruption, which was much more likely in the 90's.