- "Her mind is impenetrable; no one can read her thoughts unless she allows it. She can shield herself from all types of psychic attacks and learns to shield those around her."
- ―Entry on Bella's gift[src]
As a human, Bella considered this power a "glitch" in her head; Edward had teased her that he could only read FM minds, and that she was on AM. Her shield seems to be based on her "private mind," a place where no one can harm her. It also made her very desirable to Aro, who likes to collect vampires with special abilities for the Volturi guard. It is known to be unbreakable to psychic powers.
Three months after her becoming a vampire, Eleazar correctly describes it as a shield. With the help of Eleazar, Kate and Zafrina, Bella learns to gain some control over it, eventually coming to include a number of other individuals in her shield at the same time and protect them from psychic powers.
This proves useful in Breaking Dawn when she includes her family and friends under it, protecting them from the powers of Jane, Alec, or any of the Volturi guard. She also learns to remove it to allow mental powers to work on her, and to allow Edward to read her mind. She can manipulate it in any way she pleases, either by wrapping it around her targets like plastic or turning it into a giant dome. It also works as one-way glass, allowing the powers of the shielded vampires to work undisturbed. Bella mentions that she can sense those she is shielding, since they somehow become part of her. In the case of shape-shifters, a cascading effect has been observed: shielding an Alpha ensures the protection of the entire pack. In battle, Bella needs to be protected from physical attacks in order to keep her shield around her allies.
While her main shield is pushed out to protect others, a secondary shield also surrounds her personally, which keeps other vampires' powers from affecting her even if they get under her shield.
If a psychic power is already active, the shield can be projected to effectively block it out.
Bella initially had a very hard time pushing it out, describing it as an elastic recoil. This obstacle mainly comes from her subconscious desire to protect her mind above all else. Once she learns to let go of that desire, she gains complete control over it, able to shape it in any way she pleases (even wrapping it around her targets like living plastic), though it takes concentration to make it work properly; even to make it follow the movements of those she protects. This comes in handy when she removes Kate from her shield while still protecting Garrett as Kate attempts to shock him with her gift.
At the end of Breaking Dawn, Bella removes her shield to allow Edward to read her mind. She describes this as much more difficult than pushing it out, as it struggles to protect her and it takes all of her concentration to make it disappear altogether. Since the shield is constantly active, she needs to concentrate fully to be able to remove it even for a few seconds.
Though this shield is powerful, it has many limitations. Its greatest flaw being its inability to repel a physical attack, and an intruder can attack directly and even walk right through it. Also, if the intruder has a supernatural power, then once he is under the shield, his power will work effectively against anyone behind it, except for Bella, who has a second shield specifically designed to protect herself. If Bella is attacked while trying to shield someone, her shield will automatically recoil, leaving her charges defenseless.
Her shield is impotent against a power that is not invasive to the mind. Alice Cullen manages to get around it, because her ability allows her to sense the outcomes of decisions already made, which would probably relate more to the flow of time than to the minds of individuals. She also can't block Jasper Hale's emotional influence; Marcus's ability to read emotional bonds between people as seen when he reads the love between her and Edward; Siobhan's subtle ability to affect reality; and Benjamin's control over natural elements because it is a purely physical power.
Bella inherited this power from her father, Charlie Swan, who is also capable of warding off mental powers, though his blockage is very rudimentary and undeveloped compared to Bella's. The inheritance is presumably passed down through their similar personalities as they are both known to be loners, socially awkward and prefer to keep their thoughts and feelings to themselves.
- "The only guess I have is that maybe your mind doesn't work the same way the rest of theirs do. Like your thoughts are on the AM frequency and I'm only getting FM."
- ―Edward's theory of his inability to read Bella's mind.[src]
In Twilight, Edward explains to Bella about his ability to read minds, and how he finds her mind closed off from him. He teased her that maybe he could only read FM minds, and that she was on AM.
While neither of them understands the cause, Bella thinks her brain may be wired differently.
In the Twilight film, Edward describes the minds that he hears to her and says that she is the only one he can't hear.
In Midnight Sun, Edward describes his inability to read Bella's mind when he first meets her, noting that he hears nothing but silence, which frustrates him. He also notices that Bella's father, Charlie, has a similar mind, though not completely closed off. His brother, Emmett, also questions her closed-mindedness.
Because of this, he finds her behavior and responses fascinating, but has a hard time locating her when she isn't in eye range and must look into the minds of those who see her.
In mid-New Moon, Bella explains to Jacob Black about Edward and the other Cullens' powers, and about her closed off mind.
While in the headquarters of the Volturi, Aro has read Edward's mind and noticed his inability to read Bella's thoughts. This intrigues him enough that he decides to try his power on Bella, which proves ineffective as well. He then orders Jane, one of his guards, to try her power. This also proves unsuccessful.
The discovery of her immunity made Bella very desirable to Aro, who likes to collect vampires with special abilities for his coven.
Near the end of Eclipse, Jane once again attempts to use her power on Bella, but fails just like before.
The Short Second Life of Bree Tanner
In The Short Second Life of Bree Tanner, Bree notices Jane's attempt and failure to inflict Bella with her illusory torment, something that is unusual for her. She also notices that Jane is smiling to hide her frustration and realizes that Bella is somehow immune to her power.
- "That would indicate a rather powerful latent talent, if it was manifesting so clearly even before the transformation. I can't feel a way through her shield to get a sense of it at all. Yet she must be raw still - she's only a few months old. And apparently completely unaware of what she's doing. Totally unconscious. Ironic. Aro sent me all over the world searching for such anomalies, and you simply stumble across it by accident and don't even realize what you have."
- ―Eleazar's description of Bella's gift.[src]
Her power reaches maximum level when Bella frees herself from the subconscious sense of self-preservation, allowing her to extent the shield over an entire field and control it in any way she wishes. This proves useful when Chelsea, Jane and Alec attempt to use their powers on the Cullens and their allies, and find their powers thwarted.
At the end of Breaking Dawn, Bella removes her shield to allow Edward to read her mind for the first time. However, she realizes the removal is much more difficult than projecting it, and must concentrate wholeheartedly to be able to remove it completely from her mind. Edward, unable to suppress his amazed feelings and love for her, kisses Bella, causing her to lose concentration.
- Afton's ability is to shield himself from attackers by making them think he is invisible. His power is more limited in that he can only shield himself and can be seen through by sharp focusing. Since his power only makes others think he is invisible, this power most likely works on a mental basis.
- Fred has the ability to induce those around him with a sense of revulsion to drive them away. This power serves a great purpose in protecting him from pursuers.
- Renata has the ability to repel physical attacks, much like how Bella's shield repels mental powers. However, it is limited by making the pursuer want to go in another direction.