It's just an open mike. No detection of when you are talking or not so sometimes the road/wind noise can be picked up and transmitted through the intercom.
I've not used mine yet, but I understand that the above is how it works.
The passenger PTT is for the CB. For the passenger to be able to trigger the CB to talk. The drivers PTT is there on the front control.
Have never used mine either I removed all traces of the wiring as soon as I bought my IS

I ride a bike so I don't need to talk

and if momma needs something she knows how to whisper in my ear
