Somebody want to explain how a "2-spider gear" center differential works?? I've never seen one, and I'm having a hard time figuring out how only 2 gears can do the job.

Only design I've ever seen is 2 side spider gears & 2 un-attached spider gears, which makes differential rotation of the tires possible. I can't figure out how 2 spiders can be designed to do the job.