It's a hard microswitch:
Away from the battery terminals is normal, towards is developer.
The system will erase the volatile partition, so be careful. You shouldn't have much on there anyway:
When you boot up now, you get a scary error message:
I especially love the sad face. The computer should be happy!
Anyway, you'll have to hit Ctrl-D here. If you don't, you'll get an even scarier error message:
Well, now the computer is just aggravated! I haven't explored just how this works internally, yet, but just hit Ctrl D at the first prompt and all is well.
Now you'll have to take your picture again, and do the initial login again.
A bash shell, to be precise.
Try su sudo - and Bob's your uncle.
Now for fun stuff!!
The recovery kernel
More on switching to dev mode.