Nice! If you saw the tool used to accomplish the stamping you'd see how that happened.... Kind of like a nail gun, it should only drive one nail, but if the trigger hangs it'll rapid fire 2-4 nails real quick....
The gun may be simiar to this . Just a guess.http://www.pannier.com/stamping/heavy-duty-stamping-systems/pneumatic-stamping-hammers/
I picture a new guy on a construction site getting to run a jackhammer the first time and cant let go!!