The older pickups had a solenoid valve, when switched to the right side tank, at least I think that is the one, the switch supplies power to the valve, as long as the switch is in that position, there will be power to that solenoid valve.
When the switch is turned to the left side tank, power is removed and a spring shoves the solenoid valve the other direction. That being the primary tank.
I would always use the tank, that needed power first, that way, if there was a power failure to the switch, I would have a full tank to run on.
The 87 and newer pickumups, like the 3500 series up to I believe a 89, they have a "Motorized Valve", The switch needs to be held in the position, for the tank Yo want to switch to, for several seconds so it will give the motor enough time to switch over. When the switch is released, power is eliminated from the valve and the motor holds that position ntil it is moved to the other tanks position.
I sure wish I had My 87 K1500 GMC back, that thing was a beast.
A took a girl for a ride in it one day, Her comment was, I think this pickup is quick as a cat. I had to agree.