What's the black switch on the back of my psu?

NaniBot

Honorable
Feb 19, 2012
34
0
10,540
I'm not talking about the red switch that's on the back of some psus. Mine doesn't have that. I'm talking about the switch that reads "o" on one side and "i" on the other. Currently it's on "i" and the pc works perfectly. I want to know what it does?

My PSU - Cooler Master Thunder 700w