The following screenshot hails from the movie Sicario: Day of the Soldado, where Benicio del Toro's character puts his left index finger on the pistol as he shoots multiple rounds. I know little of pistols, but why didn't he merely pull the trigger with his right hand?
It simply allows him to shoot faster. Moving your finger back and forth within the trigger guard is a quicker movement than pulling and releasing a trigger. Try it out the next time you see "Time Crisis" or something similar at the arcade. I imagine the technique works much better with a "virtual gun" like that than with a real pistol, but then that's the only way I've tried it.