Brain Bang is used to describe how you feel after witnessing something mind blowing or seeing something amazing and out of this world. It can also be used to describe how you feel after realizing something crazy, like an epiphany on steroids.
Josh was brain banged when he realized that there was an arrow in the Fed Ex Logo and that OK is a stick figure side ways.
How did he pull that bunny out of that hat!? Man, what a Brain Bang.
Whoa! Inception gave me a crazy brain bang the whole time I was watching.