palm quickly touching your chin

We do it fast because it's a short vowel and we level our hands with our mouth to better connect lips and hands movements.

As we saw before, the motion of SOCK (a light smack on our cheek) means that we must relax the muscles of our lips and not round them. In the same way, the gentle smack of the COOK motion shows that our lips must be neutral and the vowel is quite relaxed. We slap here a different part of our face just to make both motions different.