This image captures the essence of breaking glass when the glass pane has no secured edges.

The glass being broken in this image is the side window of a minivan; the window that pivots out for ventilation. All edges of the glass are unsupported.

At the moment the spring-loaded center punch "punches", it breaks the surface tension of the tempered glass. The release of the energy of the glass causes the outward explosion of the fragments that is captured in this image.

The same effect will happen if you break glass on a window that is partially rolled down inside a door.

Be careful out there!!

Ron Moore