A non-lethal weapon (insert Jayne's scornful tone here) usually used by law enforcement, this weapon is not generally used by Alliance Military. The weapon emits a highly-focused, low-pitched sound wave with enough force to drive a big man to the ground, much like a taser would today. (Seen in Ariel and Trash.) One benefit of the weapon aside from its lack of lethality is the reduced chance of property damage on those beautiful Core worlds -- the weapon seems to have no adverse effects on non-living objects. And at close range it can't blow a door open, noting Jayne's frustration in Ariel when Mal shoots the door from the other side.
A simple 21st century equivelant of this weapon is the toy AirZooka(approve sites), which operates on exactly the same principle, albeit with a considerably lower output intensity.