Have not heard of combat breathing...will have a look.You might want to look at combat breathing. While I wouldn't advocate holding your breath underwater, I do a modified version diving. It is pretty much all I'm doing when I'm teaching... On the surface I credit it for taking me out of AFib, and underwater it will improve your sac/rmv.
IMO, it is just the act of being aware of what you're body is doing and exercising control over yourself that has the most impact. Ymmv.