Fan o Kmk, a shotgun (where I live anyway) is where you put the lit end of the joint/blunt into your mouth and blow smoke into someone else by essentially blowing through the joint, rather than sucking through it.

The only reason it gets you higher is that you're not controlling the size of the hit you're getting, so people tend to blow you bigger hits, quicker, than when you smoke it normally.

As for giving yourself a shotgun with the frozen jar method, while it may look cool and all, it probably won't get you any higher than any other way of inhaling that same quantity of smoke. Still makes for a cool party trick, though!