2nd O2 sensor?

[SaintlyCharBoy]

i installed my hotshot 2 days ago, it had a fitting for a second sensor, which would, on the stock piping, follow the cat.  

they sent me the wrong adaptor for the sensor, so
1 there is a hole in my header, about 12mm around and my car is hella loud, has 0 back pressure, poor flow, and backfires a lot

2 even though the sensor is not "sensing" any of my exhaust due to my inability to install it, the car is running fine...

any theories?
wha tis the sensor there for any ways?

i'm kind of figuring that is it just taking an O2 readig from the ambient air, so it's working, but not doing it's true intended job... and thus far i've been lucky

thanks for any insight
Sean