Imagine the RTK radio signal between the tower and your antenna as a fuel hose. Now, picture your receiver/globe as your fuel tank. As long as you have a steady connection between the two radios (your fuel hose), your tank will always be full and never emptying.

So now you're chugging along with a continuously full tank of RTK signal because you have constant supply and you approach an area where your fuel hose of RTK signal doesn't reach. The moment that connection is lost, you begin to use the fuel in your tank which is a 14 day supply of signal. Anytime that you regain connection, your tank instantly refills back to 14 days.

Ex: You're planting and you have met the criteria for RTK Extend. You've been planting for a couple days when suddenly the connection between the radios is lost. This is when it switches into extend mode, you be able to continue planting at near rtk accuracies for 14 days. So you finish the field and head to the next one and suddenly you regain signal! Boom, now you are running back on 3D RTK with no issues, but then you get down by a river bottom and you loose signal again, now you're back on rtk extend and you have 14 days again until you reconnect.