Quote Originally Posted by Ryllharu View Post
They haven't gone into detail how to kill Titans, but the answer is clear from the OP, as well as the last few seconds before the OP of the first episode. I'm not going to say it in case someone is waiting for an explicit explanation.
When I first saw it, I thought it was going for a vertical split down the spine, but on further watches they seem to be hitting the upper trapezius/neck more so in a horizontal fashion.

Arteries at the back of the neck doesn't make too much sense as far as massive blood loss goes (though it does have the advantage of sneaking in an attack and avoiding the mouth).

I'm guessing they're slashing the spinal cord regardless.