please anyone I need a +1 level explanation for my curiosity sake then what is available in this article

Why is the "nozzle" for the #NASA #MAV #SRM2 spinning? Beyond just stability.

A Rotary engine?

@AkaSci help please? Really curious as to the why

https://mars.nasa.gov/news/9448/nasa-mars-ascent-vehicle-continues-progress-toward-mars-sample-return/