Imagine going to a magic show where the magician has a box. The box is big enough for a person to fit inside. The magician invites a girl from the audience to come on stage and enter the box. The audience can see the girl's head and feet sticking out of the box.
The magician then proceeds to saw the box in half, separating the girl's body in two pieces. The audience is amazed and gasps in wonder. But don't worry, no harm is done to the girl! She is not actually sawed in half; the box and the girl are designed in a way that makes it look like she has been cut.
The magician then proceeds to push the top half of the box over to the side, and you can see the girl's torso still moving and breathing. The magician then slides the bottom half of the box several feet away from the top half.
But wait! The magician isn't done yet! He then proceeds to push the top half of the box back over the bottom half. But this time, something strange happens. The girl's head appears on the opposite side of the box than it was before. She has somehow transformed into a zigzag shape!
The audience is amazed and applauds the magician's skillful trickery. The girl then pops out of the box, unharmed, and takes a bow. The magician then takes his final bow as the audience gives him a standing ovation.
So, to sum it up, a zig zag girl is a magic trick where a girl appears to be sawed in half and then transformed into a zigzag shape before reappearing unharmed. It's all just clever illusion and trickery!