I found the assignment in the original art list from 1989: a right hand which was intended to play when a kill was scored--a thumbs up for wingman kills and a first for player kills. The feature was ultimately dropped entirely, but...

... it was still present during testing and the fist was eventually swapped for the middle finger (as a joke, not for release). QA would often use the command line cheat to alt-delete through missions, triggering the animation over and over and forever dubbing the cheat code as THE FINGER OF DEATH!