Why did Jasper's power work on Bella?

Question

Okay, thanks to RiffTrax (or because of damn RiffTrax) I've seen all the Twilight movies. One glaring error in New Moon is Jasper using his "Mood Control" power on Bella. She notices and says, "Jasper, no fair with the whole Mood Control thing."

My question is: why did it work? Wasn't so much time of the overly-long movies spent revealing vampire's powers have no effect on her? Not even the "Pain" girl Jane (Dakota Fanning) from the Volturi. Was there some explanation given in the books or is this just another weak point of a melodramatic series? It seems to go against everything else about her being a "Shield."

There is no chance I will ever read the books, so if you have read them and it's explained, please let me know the explanation. It sticks out like a sparkly sore thumb to me.

Sorry to be so negative about the movies but they seemed really poorly written, poorly acted, slow and teaching codependent behavior to girls. Still, I'd like to know what the heck's going on with this bit.

Answer 1

If I am to give my personal opinion, I clearly think its a plot inconsistency. But I'll try to salvage it just as Stephenie Meyer has done.

In the books : Twilight Breaking Dawn, Chapter 31 Talented
The following is an excerpt which takes place between Bella, Edward & Eleazar

“A very talented family, A mind reader for a father, a shield for a mother, and then whatever magic this extraordinary child has bewitched us with." ~ Eleazar

“Excuse me, What did you just call my wife?” ~ Edward

“A shield, I think. She’s blocking me now, so I can’t be sure.” ~ Eleazar

“It only works with certain things. My head is sort of… private. But it doesn’t stop Jasper from being able to mess with my mood or Alice from seeing my future.” ~ Bella

“Purely a mental defense. Limited, but strong.” ~ Eleazar

You can read the chapter summary here

From Stephenie Meyer
She has been asked this question. Specifically,

How come Alice and Jasper can affect Bella with their supernatural powers, but Edward and Aro and Jane can't?

To which she answers

This question comes up at every single signing! The answer is explained in Eclipse, but I'm going to tell you all anyway, just so there's less confusion.

Bella has a very private mind. She can't be touched there. What Edward and Aro do is clearly a mental thing; Jane, also, works inside the head (Jane doesn't actually inflict pain on anyone's body, she just puts the illusion of pain inside her victim's head. It's a very effective form of torture). Conversely, what Jasper does is no illusion. He affects the physical body, slowing the pulse and upping the endorphin levels to calm someone, for example, or raising the pulse and pumping out the adrenalin to excite them. Alice, also, works outside the mind in the realm of possible realities. She doesn't see the thought process behind the decisions, just the outcomes.

I found this answer from Meyer here. It's listed as official website of Stephenie Meyer but I dont know for sure. But her answer makes sense, albeit retrospectively

So to salvage by (unfortunate) fandom, I can draw the following conclusion

Objectively & scientifically, emotions trigger the secretion of hormones. But emotions start from the mind. Eg, fear, nostalgia, happiness begins in the mind which secrets the hormones & the entire body feels it. But I want to believe, for the sake of being an unfortunate Twilight fan, that Jasper can control the secretion of these hormones without the trigger from the mind. Thus, in the series, Bella's mood is controlled by Jasper but she is able to feel that her mood is being manipulated as her mind hasn't triggered those moods