ToME: the Tales of Maj'Eyal

From a readthrough of the code for Eldritch Combat me and some other Discord users did to see how good Ogrewielding is, it seems like the power of Eldritch Combat procs is only 50% while wielding a 2H + anything in offhand. The ability does look like it's supposed to divide the potency of each fight...

What exactly is the "extra benefit" of leveling the 4th Undead/Vampire subtype skills to 5, and how do I trigger it? Do I boost the Undead/Vampire subtype category, or what?