It’s most commonly seen when the actor is using two handguns and firing them in an alternating, reciprocating action, a la Antonio Banderas in the big shoot-’em-up bar scene in Desperado. Not to be confused with the idiotic plot-point of Wanted where Angelina Jolie teaches the new recruit to “spin” the bullet as it comes out of the barrel to make it curve around an object in front of the intended target. 
I think “cowboying” has some basis in fact, though. Shooting an SAO revolver necessitates the cocking of the hammer between shots, and if you have a gun in each hand you have no choice but to do that with the thumb. So a natural motion develops that loosens the wrist a bit and rocks the barrel upward as the hammer is cocked between rounds.
The early movie moguls looked only at trick-shooting in Wild West shows and evidently decided that the small wrist movement should be exaggerated, should include some full-on elbow action so as to be clearly visible to the audience. The result is that we have icons like Randolph Scott, Jimmy Stewart, and even John Wayne immortalized on celluloid trying to throw the bullets out the barrels of their guns. Since we know about the single-action thing, though, that still doesn’t grate as much as seeing contemporary actors do it with a pair of Glocks...