A form of school ground hazing. Originating in a suburb of Cincinnati, Ohio. (Greenhills)
A "yonk" occurs as someone sneaks up behind another person, places one hand on the back of the yonkee's neck. Then grabs their forehead and proceeds to make a neck-breaking motion. This subsequently followed by saying (or yelling) some obnoxious sound effect. The intent is not to injure anything but pride.
That new kid just got "Yonked." Hope his neck is OK.