palm moving quickly to touch your lower cheek

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.

Slap your cheek gently but quickly. This smack reminds us of the stronger smack the practitioner applies to our other lower cheek to relax our bottom before giving us an injection, because our lips must also be relaxed (and not rounded) if we want to get the sound right.