A Concussive Stun Field Generator was a small device which stunned anyone caught in the generator's radius. Its effects can be negated simply by averting one's eyes.
The Concussive Stun Field Generator was used twice by the Middleman; once in Middleman Headquarters to stun Cindy Marshall (and, inadvertently, Wendy Watson). ("The Boyband Superfan Interrogation") The second time it was used to stun a room full of bad guys and save Wendy. ("The Obsolescent Cryogenic Meltdown")