The mechanism is that the software is developed and released under GPLv2+ terms and that Microsoft doesn't completely control ownership over the software. Ergo,

The mechanism is that the software is developed and released under GPLv2+ terms and that Microsoft doesn't completely control ownership over the software. Ergo, there's no "extinguish" step. They can't make their changes a proprietary fork nobody else can use, they can't force you to use changes you don't want to get access to changes you do, and any other person making changes to Blender can use theirs.