There is a number of different boot modes available, triggered by holding down reset/power/aux(backup) buttons different lengths at restart:
Reset 5: OS Reinstall
Reset 30: Factory Reset
Reset 50: TFTP Boot
Aux 1: Boot Diag
Aux 6: User Diag
Power 5: Skip Vol Check
Power 10: TFTP Conf
Power 15: Tech Support
Power 20: USB Boot
Power 25: Memory Test
OS Reinstall overwrites the OS partition with the files in root.tgz (but keeping any old files).
Factory Reset completely reformats the harddrive and reinstalls root.tgz.
The two different TFTP modes works pretty much the same differing only in the filenames loaded. One loads kernel_test & initrd_test, the other loads kernel_conf & initrd_conf
Skip Vol Check skips fsck of the root filesystem.
The Boot Diag / User Diag modes seems unused today.
Tech Support activates tech support mode, providing access to the initrd system.
USB Boot boots from an USB stick for flash recovery or other purposes.