What exactly is the AK?

[literature_Caro]

No: HPFGUIDX 143850

I think this was discussed a lot around and it could be that the thing
I am searching for has already appeared here:

In book four the dead Riddles are all described to have the expression
of utmost terror on their faces. This was of course due to the AK.
But what with DD, who was also AKed in book six. His expression was
written as if he was asleep, except for the weird angle of his limbs.

Does this mean that the face of the killed ones expresses their
attitude towards death? (DD said in book one that dying is like going
to sleep after a very long day!) Or does he look like this because he
agreed with his death (or even pleaded for it) to save Severus Snape?
What did the Potters look like after their deaths?

I am interested in your thoughts because I cannot make up my mind
about it and I think it all possible.

Yours
Caro