how come a small piece of porcelain can break a car window what happens?

Answer 1

Porcelain is harder then glass. If that piece of porcelain has a sharp point and that point hits the window, the safety glass will shatter.

I also saw a report about thieves in S. Africa you use old sparkplugs. They break the end off and remove the electrode so the porcelain will contact the glass. They walk through traffic and when they see valuables on a car seat they hit the window with this homemade tool. The side window just shatters like being hit with a glass hammer then they snatch and run.
The new anti-snatch and grab is applying tint or window film to the inside of the window. The glass spiderwebs, but stays inplace due to the plastic film.

Answer 2

Auto glass is designed to shatter upon a sharp impact.