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.

Yeah the days before journaling filesystems, a moment to consider your actions as fsck did its best to clean up the mess you made.