The person gently smacks the head or forehead.
Realization The person notices a mistake or realizes that he/she forgot something.
“Oh no, what have I done.”
The gesture often occurs with honest people. The slight blow to the head is like a symbolic self-punishment. The blow shall re-establish order. A blow to the back of the head or very vigorous rubbing shows that the incident bothers the person.
This is one out of 63 gestures from the body language trainer.