i have an ls-turbo and was wondering where the extra inlet goes

 

you can leave it off I have mine reading atmospheric. I have a vacuum line hooked up the the one in the middle and the one below it I have open. ive heard some people hook another vacuum line up to that if your valve is flutter but that usually can be fixed by loosening or tightening it.

 

Now if you had searched.... j/k

THe lower port connects the lower half of the diaphragm to boost (pre-throttle body). When you let off throttle, vac is introduced above, and boost remains below. The valve is opened faster, and with less boost. This helps with flutter and momentary compressor surge, but at the expense of holding higher boost. It should be a non-issue, since originally Greddy recommended to use the Type-R BOV above ~15psi anyways.

It can also help with keeping it closed at idle ever so slightly, since the FMIC's pressure drop can create an ever-so-slight vacuum in the charge pipes, which holds the valve closed a fraction farther. Enough to theorize about, but not enough for practical use...