Not talking about stealth kills. Finishers that are activated in the middle of a melee. Is it a % chance? Do you have to time your strikes? I'm a bit confused on this matter.

I have no idea. Some people are complaining about seeing them all the time, but I've only seen them SIX times in 30 hours. I actually want to see them more often.

It's random.

I went two hours without seeing one, and then did five in rapid succession in the same bandit camp.

it takes a % chance when enemy health is low enough for a kill blow