It's not uncommon for NFL teams to bring in special instructors during OTAs or training camp. The Denver Broncos brought back former Bronco DeMarcus Ware to work with pass-rushers. The Patriots have famously worked with Navy SEALs.
The Cleveland Browns went in another direction, bringing in MMA legend Chuck Liddell to work with, well, everybody.
Based on this video, it seems like most of Liddell's instruction would be useful for the defensive line, as there's a lot of clubbing and ripping involved. But he also said that one of the most important things he worked on was teaching players about their mindset, which can really apply to everybody.
The most important thing about this, though, is that there better be a bunch of time devoted to these sessions on the next season of Hard Knocks. We need to get all the minute details of how Liddell helped the Browns, just in case they decide to not be so Browns-y next season and actually win some games.