After 4 hours of going though dungeons and only using standing power attacks on master difficult, not a single decapitation happened.
This is not bad luck, this is a bug, maybe some files are missing, i dont know...

I just tried removing the perk with player.removeperk and re-adding it with player.addperk, tested for around an hour, no decaps...
Im using sword on right hand, shield on left hand. Hold right-click while not moving, standing power-attack, i know that stuff...

Can someone help?

While it sounds like a dumb thing, try lowering the difficulty and see if it happens? I have savage strike and it procs fairly often but I only play on expert so who knows.

Have you tried it without a shield? I've been able to decapitate a few times with a sword in my right hand and a spell in my left. Maybe there's a triggering issue when you do a power attack from behind a shield?

Also if you are dual wielding I have NEVER gotten a proc while dual wielding so it may be something to do with that.

I have yet to see a decapitation and I've had it quite a while. This is with magic in the left hand,

Tried it on Novice yesterday, no luck...

And i've tried it with spell/sword as well... :c

Is it just a finisher? Just a cosmetic eye candy? Or is it a chance to one-hit-kill someone? Does it depend on your positioning?

Since apparently I'm the only person to have decapitated someone in this thread, it is a finisher. Though sometimes finishers appear to be tied to critical hits as well. My step brother has been able to do it as well, but he dual-wields and does it by the double power-attack.

Maybe you have to be perfectly in front of the enemy? facing him? Well, if its just a finisher, it doesnt affect my damage, right? So i can just forget about it?

I've personally never had this happen on an enemy with 100% health unless they were already destined to die from the first hit. Anyway, I get these all the time with a 2H sword. The cutscene animations seem really fidgety and I don't know the logic behind them since I've actually been moving and gotten decapitations before as well. Sometimes I'll get several in a matter of 25 minutes and other times I'll be playing for hours with nary a one.

I think it's just eye candy. It is amusing to watch the head roll down a hill, though. The bad thing is that you can't pick it up. It just kind of stays near the ground as if it were stuck.

If its just eyecandy, ill just forget about it, i guess... the +25% more damage to power attacks is still worth it.

I use a 2H sword on Expert with the perk and I've managed to decapitate an Adept Mage right off the bat. I'm not sure how it works I agree the +25% is worth it!

I've had it proc plenty of times on my Warrior with Axes and Maces.
Haven't tried Daggers or Swords yet.

Readily chop heads off with swords, though in these circumstances I'm not sure if the strike would have been fatal anyway.
Given the amount of melee damage my buffed sword does I don't think the decapitation is doing any extra damage. It looks cool though