How to use Swann's Immortality Protocol?

In Starcraft II, when using Swann in Co-op, how do you activate/use the Immortality Protocol? I have earned it, but I'm unsure what I have to do to get it to work and I can't find anything on the internet.

To use Immortality Protocol, the unit needs to be destroyed.

When the Thor unit is destroyed, if you have the Immortality Protocol, you will have a 400 HP Unit that you can select, and rebuild the Thor.

Note: Google Search for "Immortality Protocol" will tell you everything you need to know on the first hit. The question is extremely poorly researched, and it seems like no attempt was made.