When you turn the gain up on the DVX what actually happens?

Doses it open the lense, change the shutter speed boost the light digitally?

Like most cameras, it electronically boosts the signal - thus noise increases.

Yeah, it does the same thing on all cameras, most of them go upto +18dB gain too (even my $350 panasonic does). On most cameras you can go upto +9dB gain without seeing TOO much visible grain, but if you try to work with that footage in editing, doing some "correction", the noise will become more apparent. Oh yeah, the IRIS is open all the way when your in gain too.