A shield refers to both a self-defensive talent and the vampire showing it. Since no two human beings or vampires think exactly the same way, no two talents are identical; accordingly, different shields work differently, though with the same end: protecting the bearer.
Two different types of shield are recognized: "mental", which repels purely mental attacks but fails to be effective on physical ones, and "protective", which foils a physical attack by making the attacker forget its actions, intentions and motivations.
Mental shields
Bella Swan: a mental shield
This type of shield blocks any and all powers acting on the mind, not necessarily malevolent ones. Two people show it in the books: Bella Swan, who has it in its most advanced and developed form, and her father Charlie, who shows it in its most undeveloped and rudimentary form.
As a human, Bella considered this power a 'glitch' in her head; Edward had teased her that he could only read AM minds, and that she was on FM. 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.
Three months after her becoming a vampire, Eleazar correctly describes it as a shield. With the help of Kate and Zafrina, Bella learns to gain some control over it, eventually coming to include a number of other individuals at the same time.
This comes 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, as to allow Edward to read her mind. While her main shield is pushed out to protect others, a secondary shield also surrounds her personally. It also works as a one-way glass, allowing the powers of the shielded vampires to work undisturbed. Bella mentions that she can sense who she is shielding, since they somehow become part of her. In the case of shape-shifters, a cascading effect has been observed: by shielding an Alpha allows protection to the whole pack.
Bella initially had a very hard time pushing it up, 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 make it wrap around her targets like living plastic), though it takes concentration to make it work properly, even make it follow the movements of those she protects. This comes handy when she removes Kate from her shield while still protecting Garrett.
In battle, Bella needs to be protected from physical attacks in order to keep her shield around her allies.
Limitations
Though this shield is powerful, it has many limitations. For instance, it cannot work against a physical attack, allowing for an intruder to attack directly.
Renesmee Cullen is the only individual capable to penetrate it, placing thoughts and images into her mother's 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 mind of individuals.
Bella also can't block Jasper Hale's ability of emotion manipulation; Marcus's ability to read relationships; Siobhan's ability to affect reality; Benjamin's control of natural elements because it's a purely physical power; Charles's truth detection as shown when he was able to tell Garrett was telling the truth even though he was still under Bella's shield; and James's tracking sense.
Protective shields
- "If someone approaches her ― or Aro, as she is always close beside him in a hostile situation ― they find themselves... diverted. There's a force around her that repels, though it's almost unnoticeable. You simply find yourself going a different direction than you planned, with a confused memory as to why you wanted to go that other way in the first place."
- ―Eleazar on Renata's power[src]
Protective shields have the power to deflect any and all kinds of physical attack, by making the attacker temporarily forget her or his intentions and motivations and go the other way.
Relatively little is known about this power. Renata, of the Volturi Guard, shows it in protection of Aro, and she needs to be physically in contact with him. On occasion, she includes Marcus and Caius in her shield. Eleazar has mentioned that she can project her shield several meters out from herself to protect those of her priority.
Limitations
This power is more restricted than Bella Swan's. As, like most abilities, it is mental, it does not influence Bella and those she protects.
Mental invisibility
The power to make the user mentally invisible to others. Afton is the vampire who possesses this power.
It can also hide someone by the user directly standing in front of a person.
Limitations
It has been explained that this power is very limited and much weaker than a real shield in that it can only "shield" the user from people who lack focus. If someone has a strong ability of focus, he can easily see past the invisibility. Since this power takes place in the mind, it most likely has no effect against Bella's shield.
Control
- "The shield blew out from me in a bubble of sheer energy, a mushroom cloud of liquid steel. It pulsed like a living thing - I could feel it, from the apex to the edges."
- ―Bella Cullen[src]
Experienced shields are also able to shield others. This is called "projecting". Depending on the level of control the bearer has with their gift, shields can project vast distances and mold their shields to any shape they want. During the Volturi confrontation, Bella was able to shield all of the witnesses the Cullens had assembled. She was also able to shape her shield to only protect Garrett when Kate shocked those trying to stop her from attacking Caius. Eleazar says that Renata can also project her shield several meters out from herself, protecting Marcus and Caius when needed, though Aro is her priority.
Projecting is proven to be difficult to learn for shields, as Bella says she had difficulty letting go of that invisible, subconscious need to protect herself.
Possible shields
The following are not formally named as shields, but it is speculated that they may fit the description:
- Fred, with the gift of illusionary repulsion. Since Fred's ability is mental, it is likely that, as with Renata, Bella Swan is immune to it.
- Victoria, with a talent for self-preservation that allowed her to find the escape from danger in a given situation.