Lemme see if I recall the explanation: With both tanks open and switch pointing to one tank, that tank is the "service" tank. The other is the "reserve." Initially, the indicator will be green. When the service tank is depleted, the indicator will change to red and the regulator will draw from the reserve tank. You go check, see red, switch regulator over to the formerly reserve tank, indicator goes green again and you go get empty one filled. That's in a perfect world.
Here in reality, the service tank will draw down to about 1/4 and then the regulator will start drawing off the reserve. The indicator will remain a jolly green. You'll go out there, see a frost line still on the service tank when you think it should be empty, lift the reserve tank, and find it's empty.
Neighbors get ears burnt from your tirade.
Children cry, women tremble, weak men hide.
Moral of story: open one tank at a time and check it frequently when running off propane. Or find out who has 24 hour propane service.