This is when a chick "pips" (pecks a hole in the shell of the egg) and then turns itself around in the egg breaking the egg as it goes around and "un-zips" the top of the egg so it can get out. That is the best way I know how to explain it! Especially at 11:30 PM!
The pip is a beak sized air hole. The zip is a straight line that comes off the pip, that makes the hinge for the chick to escape the egg. Sometimes a chick will pip the airhole, then not zip for hours (during this time it's often absorbing the last of the yolk).