Ok I have researched the matter and here's what I've come up with. A multi-shot is caused by the Barrage ability. It is given to a unit and has it's damage, range, cooldown, missle art, and missle speed to the same as the Hero's attack. Barrage is used whenever the Hero attacks, making it look like the Hero is shooting multiple projectiles. Say, if you had the Dark Ranger with the Black Arrow ability. When you have that on autocast and the Dark Ranger attacks someone, Barrage will not activate because the Dark Ranger is not attacking. She is using the Black Arrow ability. So when on autocast, the Dark Ranger can't use multi-shot.