How do I do "finisher moves" in The Witcher 2?

Solution 1:

As sjohnston stated, if you manage to stun an opponent, you will be able to perform a so-called finisher move on him.

The probability of landing a finisher move depends on the chance of causing a critical effect, in this case, stunning.

You can view your chances of causing critical effects in the character panel (enter meditation and select "Character" from the radial menu)

The odds can be raised by getting or improving the appropriate skills, acquiring new mutations, drinking potions and using oils.

Solution 2:

I have found that enemies are sometimes stunned by the Aard spell. If you attack them while they are stunned, you get an instant-kill with the more cinematic animation.