Yeah, the quest skill wouldn't be that practical/fair, I imagined it as only ever working when the enemy is at least writhing if not convulsing, a little bit like disembowel, but even harder to pull off. I just thought it would be really cool.
It is most definetly predicated on the assumption that your opponent be leaning forward after the parry however. I always thought it would be a badass quest skill to have added on to cross where after the kick, you bring your swords out to your sides at shoulder level, and as your enemy recovers, dazed from the blow, you swing them together, decapitating him/her.