In the One-handed perk constellation (skill tree), there's a perk called "Savage Strike". The description claims that "Standing power attacks do 25% bonus damage with a chance to decapitate your enemies." What is the probably of to decapitate your enemies?
Tho the 10% chance sounds about right,here is where it gets confusing,ingame you can use console command "show/set decapitationchance" which shows you the value of 40%,to be even more confusing the 40% chance is the same whether you are lvl 1 or level 60 with savage strike perk.
Which means that perk savage strike and finishing move decapitation work independently.Which does make some sense since savage strike indicates that it must be Standing power attack to trigger decapitation,where in contrary people say that they managed to get decapitation while sneaking,crouching,moving,without power attack...
Telling you where exactly those 10% fit in to the finishing moves would be just me guessing. You should read more about finishing moves and see for yourself.