Tempered glass is on high pressure and tries to explode but the outer part of it is on high tension so that it doesn't. If you apply pressure to it the tension absorbs it so that the glass can take much more pressure than normal glass. But when you apply more pressure than there is tension on the surface the inner forces will break free and shatter the glass. With these little hammers and so on you can create a lot of pressure in a very small area and that's enough to release the inner forces. You could compare it to a flour filled balloon or something. You can try and punch a balloon as hard as you can it won't get destroyed but the moment you pinch it with a needle it explodes.