Fujifilm GFX 100 Pixel Shift - How to Create 400 Megapixel Photos

At the end of November Fujifilm released Firmware Version 3.00 for the GFX 100 medium format camera. Not content with providing 100 megapixel images straight out of the camera, Fujifilm have added a brand new feature called Pixel Shift. This is a special Multi-Shot function that can be used to create 400 megapixel images! How does the GFX 100 with version 3.00 firmware do this?