UTC  You literally record it on a separate track on the tape or these days on the digital recorder so you can remove it Each mike comes through a sound board and you can add or delete it Generally before digital you had eight tracks on a tape four before about  I think the Beatles Abbey Road was the first professional album to use fourtrack so you record different mikes on different tracks If you keep a track free you can add a laughter track later I would imagine it is also used for dubbing if you record actors voice mikes on a separate track There must be some crossplay that it will end up as an echo on other tracks I am not an expert      January