I noticed this last week when I was farming H25 Firelands but didn't know how it worked at the time.
When you fight Major Domo you will get a little bar that will fill up and give you a buff called concentration which when you scroll over it will say +% dmg up to 100% as well as a a "Legendary" How I Met Your Mother reference in the tooltip.
For me it doesn't fill up until after I kill Major Domo.The buff shouldn't stay after the fight. I noticed I had been stuck in combat which is what lets you keep the buff through Ragnaros.
I know what keeps you in combat but not how to get it every time. Right before Baleroc there is a a group of Flamewalker Animators that channel on a Unstable Magma. I normally skip these the way that trash works is you kill the Flamewalkers that are channeling which will make the Unstable Magma attackable. Sometimes after you kill them the Magma never becomes attackable and stays where it is keeping you in combat. So when you kill Majordomo you get to keep the dps buff. I'm assuming you have to kill them to keep the Magma from becoming active. I will update if my findings prove otherwise in future testing. Hope this helps and is useful for someone.
I was able to recreate this affect on an alt. Good luck and hope it helps some what as you solo rags.
![]()