When I first tried it, I thought it wasn't working until you realize there is no animation associated with it, so how do you know its working? With floating combat texts while no visuals come with it, you will see exactly 4000 damage pop ups dealt to other targets, since its Oblivion damage it can't be resisted and I don't believe it can crit either so every time it procs you see exactly 4000 damage if its maxed out. It's most notable for me in Delves and Public Dungeons where mobs are tightly packed together in groups of 3 or more.

As for who it works best, I would say magicka builds as you can get triple elemental debuffs with class skills doing one elemental damage and an overcharged destruction staff with the third element enchanted. So DK does Fire damage skill + Lightning Staff + Frost damage enchant for optimum proc chance; that's my opinion of course. It can work with Melee damage as well, best combo is having a bleed and poison effect as the damage over time increases the frequency of applying a status effect with each tick.