During football, children in the process of playing strike each other with balls much more significant than during physical punishment. Taking a wet ball with your head is many times harder than any cuff from the heart. By itself, a blow during physical punishment is not the cause of any mental trauma.
Physical punishment can cause psychological trauma due to moral blows, due to blows to sore points in the child’s soul.
If a child is accustomed (or taught himself) to be frightened of punishments, turns off his head during punishment and only shrinks, punishments are meaningless. He fought, you spanked painfully, and his body shrinks, his eyes are frightened and meaningless — cause harm, possibly inflict mental trauma, and the issue will remain unresolved. Therefore, it cannot be punished.
And if they slapped, and the child cries cheerfully and fully understands, then at least it is not harmful. Another question is how this solves the problem and whether it is possible to find a more acceptable variant of pedagogical influence.
In the film The Miracle Worker, teacher Annie Sullivan hit back when her pupil Helen Keller went hysterical, defending her right to tyrannize loved ones. Annie saw that Helen was quite cheerful, fighting for her power and mental trauma in this case does not threaten.