Definition of sidewall tear:
Zipper ruptures (also known as sidewall tears) occur in the upper sidewall of the tire as it fails and “blows out” of the sidewall and appears like a large tear around 10-12 inches long typically but can be as large as three feet! The tear itself looks like a zipper with teeth alternating between the sides of the tear.
The tear has the teeth like appearance because there is a sudden transfer of load as the air pressure is evacuated which spreads from one point but evenly on the top and bottom of the initial location as it spreads horizontally. Only one side of the whole tire ever ruptures.