The way I've found Startup keys to work is this (and for the purposes of this explanation "Enabled" means Monitor Status On):
When you press and hold a number key (Startup Configuration Key) during scanner power up, all the Favorites Lists assigned that startup key have their Monitor Status set to On during the power up. If the status was Off before the power up, it gets changed to On. (If the status was On, it stays On.)
Additionally, only those Favorites Lists associated with the Startup Key are loaded into the memory during power up. That's because the scanner only loads those FLs whose Monitor Status is On (and meeting location parameters if applicable).
Anticipating your next question: The startup key process does NOT appear to alter the status of a Favorites List Quick Key, however. Off stays Off, On stays On.
For a long time I resisted the use of Startup Keys, perhaps getting too old to learn something new. However, Startup Keys are beginning to appeal to me. I have several scanning scenarios (Immediate Home area, Home area plus 50 miles, Up North and Down South). Each one of those scenarios has its own Startup Key. I also assign ALL Favorites Lists to Startup Key 0, so if I get lost or something isn't working quite right, I can get everything going.
I'm beginning to favor this for two reasons. One is the scanner power up time is much less - I have numerous FLs, many of which do not need to be loaded (for example, when in Kentucky I don't need Minnesota's loaded).
The other reason is I do not have to swap out SD cards. That gives me a few more extras for programmed spares. I had different cards for different Profiles.