There is a tiny switch on top of MSI R9 270x. What's that?

It's so you can have two BIOS on the same card. One could be for the normal BIOS and the second could be an overclocked version of some sort. If the second, modified BIOS happens to mess up you can flip the switch to use the first, useable BIOS.

It's basically a backup feature.
 


Oh, okay. Is it okay to change the bios while the power is ON?
 


Okay, I got it. Among those two bios, which one is overclocked? I used MSI afterburner to find if the default settings change when the switch is flipped to the other side and to be sure, there are no changes. I don't think that one is for normal clock speed and the other one is for overclocked version.