I’ve got a laptop that uses an Nvidia 310M/Intel hybrid graphics. It’s an interesting (and ridiculously flawed) system. One of the flaws is that, when my computer auto-switches from GPU to GPU, my screen brightness gets changed. I can’t figure out how to get this to stop, as it seems to be part of the Nvidia drivers, Nvidia’s control panel is the GUI equivalent of the Tunguska event.
check your energy saving settings, it’s set there
btw, I have an optimus notebook as well and it works flawlessly