That's a good question. I think usually FM wants us to keep eye contact during the solo you're doing and communicate that you want to pass the song then (it is actually very difficult for me to do this, especially when trying to play a lead, but I try to look up every once in a while). The actual passing of the song happens after the end of the solo you are playing, of course.
Or alternatively, your group could arrange it all before you come in to play the song. Oops! Did I say that? Must be temporary insanity