you're just talking about a separate mic for technician communication. I would assume the sound technician was positioned out front.... but why in the living hell they'd place the mic where you could see him using it is beyond me. Like Steve said:
We have seldom played with a sound system monitored by a sound technician... but we have done that and we simply used hand signals. About all you can adjust competently onstage is volume and that's pretty easy to communicate by hand.... as for sound quality.... the sound you hear on stage is completely different than what the audience hears. That is set during sound checks long before the audience even arrives.
Balance between the instruments and the vocals are hard to set once you start playing (without a sound man)... BUT... very much needed. In my experience, sorry Steve, guitar players during the gig will play at 10 times the volume they play at during sound checks... thus radically changing the balance once the gig starts. In all fairness I've seen lots of drummers do that as well.
There are a couple of songs we play that our lead guitar players LOVES... so when those songs come up I prepare myself because it's going to be LOUD.
Which reminds me... Steve, you go by preset song lists during gigs, right? We don't do that. We have before but it doesn't work for us...we're not a 'listening' band; we play for people to dance too...and every audience is different so we decide on the fly... sometimes guessing right, sometimes no.