1. The whole thing is in 2 parts, minus the battery; the shotgun, and the sensor and associated electronics.
2. It utilizes a PIR sensor (Passive Infrared) in order to detect when a person has stepped in front of it.
3. It also has a remote trigger function, allowing the user to time exactly when the whole thing fires.
4. It is all powered by a 12 volt, 7.2 amp/hour gel cell battery
I'm sorry but this is not a tutorial on how to build one, it is far too time consuming and costly to really be considered a feasible tactical advantage.
Alright then, so here is the schematic for the electronics. I am not really an expert in drawing these things, so feel free to point out any mistakes.

So here is the thing in action:
http://youtu.be/Q8atrwWokEQ
So here are some pictures of the internals and the shotgun and stuff:










