They aren't saying as far as I can tell, except that it receives an optical signal at the rear. Some people are assuming it has fins, but none are visible in diagrams. Beginners can hit moving targets at extreme ranges.

http://bulletin.accurateshooter.com/2018/07/guided-50-calibe...

https://www.darpa.mil/news-events/2015-04-27

Sandia National Laboratories did something similar, but with a non-standard rifle and using a laser target designator. Russia is attempting to do something similar.