Switch #1: Between battery & IGN line on Opus. When on, the opus acts as normal. It turns on and off based on the position of the key. When the switch is off, the Opus never gets the signal from the car that the key is on so it never sends the signal to the computer to boot up. [When the opus gets power on the IGN line, it just simulates pushing the power button on the motherboard, and pushes it again to shut down]
Switch #2: Between the battery and the 12v line on the Opus. This kills the line carrying all the work-load electricity to the Opus. Sometimes something locks up so that pressing the power button wont work. THat's when you need to do a hard reboot. When I flip that switch, it's the same as pulling the power plug from the back of the computer. It's a last resort, but since you cant hold the power button down to do a hard reboot, it's come in hand a few times. I also flip that switch when leaving town for a while. I usually leave the computer in standby, but when leaving town for more than 5 or 6 days, I do a full shut-down and kill that switch. A bunch of my USB devices dont fully turn off when the computer does. For more info on that, do a search for the Epia USB issues.