Mental shield is Bella Swan's raw supernatural force-field that blocks any and all kinds of psychic powers that act on invading the mind, though not necessarily malevolent ones.
It is known as one of the most powerful supernatural powers in vampire history due to its ability to withstand the strongest powers of the Volturi.
As a human, Bella consdered 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 and is able to shape in any way she pleases, 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 off, 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 a vast range of limitations. Its greatest flaw being the inability to repel a physical attack; an intruder can walk through it directly to attack. Also, if the intruder has a supernatural talent, then once they are under the shield, their power will work effectively against anyone behind it as well as those outside of its protection, except for Bella, who has a secondary shield specifically designed to protect herself. If Bella is attacked while trying to shield someone, her shield will automatically recoil, leaving her charges defenseless.
Renesmee Cullen is the first individual to penetrate the shield, as she can place thoughts and images in anyone's mind, including her mother's.
Her shield is also impotent against a power that is not invasive to the mind. Alice Cullen's clairvoyance allows her to see the outcomes of decisions already made, which probably relates 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, Siobhan's subtle abiity 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.
Edward found himself unable to fully read Charlie's mind, but he dismissed it very quickly, believing that he was just slow on the mind.
In Twilight, Bella realizes Edward Cullen's ability to read minds and also that he can't read hers. He explains about this ability, 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. He also constantly tells her how frustrating it is not reading her thoughts.
In the Twilight film, Edward describes the minds that he hears to her and says 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 their 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 New Moon, Bella explains to Jacob Black about Edward and the other Cullens' powers, and about her closed-of mind.
While in the headquarters of the Volturi, Aro reads Edward's mind and notices his inability to read Bella's thoughts. This intrigues him enough to try his own power on Bella, which proves ineffective as well. He then orders Jane, one of his guards, to try her more offensive power. This also proves unsuccessful, much to Aro's satisfaction.
The discovery of her immunity made Bella very desirable to Aro, who likes to collect vampires with special abilities for his coven. Despite the invitation, Bella declines joining the Volturi.
Near the end of Eclipse, Jane once again attempts to use her power on Bella, but fails just like before.
Bella later asks Alice to explain why she can see her future and Jasper can affect her emotions when other vampires' powers have no effect on her. Alice answers that her power is to see the future affected by people's decisions, but not the thoughts and reasons behind them; and Jasper can actually manipulate people's emotions.
The Short Second Life of Bree TannerEdit
In The Short Second Life of Bree Tanner, Bree Tanner 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.
In Breaking Dawn, Bella finds out about her gift via Eleazar and Edward three months after becoming a vampire. With the Volturi approaching to end the Cullens, Bella spends most of her time learning to fight and to harness her shielding power. She learns to extend her shield from herself and protect others from similar abilities.
Her power shows maximal potential when Bella frees herself from the subconscious sense of self-preservation, allowing her to extend 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 let Edward read her mind for the first time. However, she realizes the removal is much more difficult than extending, 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.
She expects to be able to improve her removal until she learns to someday be able to pull it back completely.