For those wondering how center punches work, or them "going off", I took a photo of mine taken apart.
When you push the point(A) against the glass, it compresses the spring and pin assembly(B) down against the edge of the cup(C), which compresses the bottom spring(D). Eventually, the pin(B) slips into the hole in the cup(C), which cause the cup to slide up a small distance until the closed bottom impacts the pin. This provides a small concussive force which is passed along to the tip and into the material it is pressed against, be it glass, steel, wood, etc... You shouldn't "slam" these against the glass you are attempting to break, because it may glance off. From what I was taught, you choose a area near the bottom edge of the glass near the door, and press/push until it "fires". This stops your hand from going through the glass,possibly injuring yourself, and also makes sure that the point is making steady contact.
Trust me when I say that you would know when you are pressing up against the tip way before it builds up enough pressure to spring, so you have more chance of being injured by keys in your pocket than a center punch firing.
