"Ha, ha! Good as new!" -Thor Pilot, after an SCV repairs his walker shortly after the "Immortality Protocol" is initiated
I believe it's called "Immortality Protocol" because it gives you the ability to extend the life of your Thor when you have SCVs nearby when it's been knocked out on the first round (If your Thor has been killed, it will turn into a scrap heap with HP until the enemy decides to get wise and attack the scrap heap, as long as that scrap heap has HP, you can still repair it to turn it back into a Thor). The idea is that the Thor is so expensive, that you're going to want to keep it well-maintained so you're going to want to take an SCV or two with you to make sure that it stays well-repaired even after the protocol initiates (it always initiates after the Thor has been destroyed in its walker form, but it will not apply if it has been destroyed as a scrap heap).