When a human head is compressed in a manner which causes it to pop.
A man crosses a street and trips in front of a car making a right turn. His head lies perfectly in line with the cars front tire and is slowly ran over, causing a quick pop as the tire reaches the peak of the head.
Onlooker: "ah man!! Did you see his head pop?!"
Onlooker 2: "ahh jeah, that head popped hard!"