"If it doesn't incapacitate, why are people supposed to be buying them?"
....................................................................................................
The simple answer is that the stun-gun manufacturers want to make money, and will say anything to convince consumers that there is a benefit to spending money. ***************
I used to carry a stun gun while in college. I had my brother stun himself and it didn't produce much of a response really. He was standing and it didn't take him down. He likened it to something slightly worse than a light socket shock and was laughing immediately afterwards. Not too comforting when I was using it to protect myself. I never felt that it would completely incapacitate anyone I needed to stun. I felt it might give me a chance to catch them off guard so that I might be able to escape. I was also very afraid that it would end up being used against me. I always had it in a case that was attached to my wrist so that it would be harder to get away from me. It shocked through the case.
Movies portray so many things to exaggeration. Cars rarely blow up when crashed, just as people are rarely knocked unconscious when stunned with a stun gun. Understandably it would be worse on a six year old who is being held down and repeatedly shocked.
Even if her physcial response to the stun gun was not to be knocked out, she could have fainted from the stress. Think he probably slapped the tape on her mouth, wrapped her with the blanket, stunned her and took her downstairs. So even if she didn't faint or wasn't knocked unconscious, the taped mouth and wrapped limbs would have muffled any movement.