here in northern california, there is no setting on the fastrak. They give you the brick of plastic with no buttons on it. Every time you travel through a toll booth, they take a picture regardless of whether you have a fastrak on your car or not. The picture is sent through a processing facility where someone will review it.
If you have fastrak and the signal goes through, your picture automatically pass and no one will review the picture. If for some reason, your fastrak didn't go off, your licentplate is compared to the fastrak registered data base. If you have your vehicle registered, and forgot your fastrak that day, you'll get charge with the appropriate amount charge to your account, which is usually 5 bucks, instead of a ticket sent to your home, which is 45 bucks. If you don't have a fastrak and your vehicle was not registered then your picture is reviewed, you'll get a ticket sent to your home.
For the carpool lanes: same thing. However, there is one more step in the process: even if your fastrak goes off, there is a chance that your picture is process, i think 1 out of 5 pictures or something, and they check how many are in your car. If you don't have 2 passengers, and you have fastrak, you have a 20% chance of getting a ticket mail to your house.
Introducing a switch into the fastrak system seems weird to me? anyone can rationalize why they would want this? Maybe they want to cut back on people who review pictures? Maybe, I don't know.