To surprise a ninja and get smacked by his spring-loaded erect penis as a result (or wang). In everyday usage, a term used to describe the result of colliding with a ninja or someone of ninja-esque qualities, resulting in disorientation and humiliation.
The ability to ninja-wang or to cause ninja-wangification or to possess a ninja-wang is reported to be an ancillary reason for a ninja's inherent stealth.
"Ninjas don't just walk slow to not be heard ..." -- John S.
Quote from a waking gaurdsmen:
"I didn't see him coming. I just walked around the corner, and the guy ninja-wanged me."
Quote from a Master:
"Nevermind the pebble. To truly be a master, you must possess the ability of the Ninja-Wang."