How do you boot a V880 off a different disk?

Normally, all I ever have to do is put the disk I want to boot from in
disk slot 0, turn the key, and push the button. I've never done it on a
V880, and apparently it's different.

I made up an alternate boot disk on the disk in slot 2. Shut down the
box, and swapped the disk in slot 0 for the disk in slot 2. Brought the
system up, and it booted from the disk in slot 2!!!

W-T-H?