Shield

The shield is a gameplay mechanic that was first seen in the game BioShock Infinite  . The shield is composed of a magnetic-repulsive field, that increases the amount of damage that Booker can take before succumbing to injury.

The shield allows Booker to take damage without affecting his actual health. The effect lasts until the shield is depleted, after which, any damage is subtracted directly from Booker's health bar.

During gameplay, the shield is shown as a yellow bar that is located directly above the health bar. With the exception of color and the respective symbols (the health bar is symbolized by a heart, and the shield bar is symbolized by a shield)

When the shield is active, all damage taken is absorbed directly by the shield. Any damage the shield takes manifests in the form of yellow cracks all over the player's screen. The closer the shield is to breaking, the brighter the cracks are. Once the shield has taken enough damage, it will "break", manifesting as a pane of "glass" shattering (the sound and visual effect mimic glass, though the sound and shattering effective are a result of the shield itself)

Note that the shield does not negate damage from a single attack beyond its threshold. In other words, a massive atttack can deplete the shield AS WELL as subtracting damage from Booker's health i.e. if you shield is low and a Handyman or explosive hits you, the shield will break AND Booker will take health damage as well.

If the player remains in cover or manages to avoid damage for 5 seconds, the shield begins to recharge, taking an additional 1-2 seconds to reach its maximum charge. The visual effect of the shield recharging is the opposite of the shield breaking: bright yellow cracks form over the screen and gradually get dimmer until they fade away, as the shield recharges.

Booker is given the shield upon first entering the Blue Ribbon restaurant. Upon entering, he finds the only occupants to be none other than Rosalind  and Robert Lutece  meeting them in the restaurant,. Rosalind offers Booker a yellow phial on the tray she is holding as an "aperitif". Upon consuming it, the shield makes an initial charge to full capacity.

Booker inquires as to what just happened and gets two responses.The first of which is a comically serious statement from Rosalind voicing her incredulity that it "didn't kill him" (him being Booker). The second response is a more serious one from Robert, who states that "...a magnetic-repulsive shield around ones body can come in handy".