palm stuck to your chin, suddenly moving outwards

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.

Put your palm horizontally with an edge touching the skin between your chin and your lower lip, then move it forwards with a sudden jerk. The position of the palm, right below the middle of our mouth, is a sign that this vowel is quite central, but more open than the schwa (the most central vowel possible)