You reinforce one of your arms with magic, allowing you to use it as a shield to block blows without injury.
You get a bonus to AC while that hand is free.
This bonus is meant to be used in place of a shield. It does not stack with AC bonuses from shields or weapons.
You can only receive the benefits of one Arm Shield at a time.

Suggested Backfire: Your arm falls off!
… just kidding. Your perception is trapped in an illusion and you are convinced that your arm has fallen off until you rest, and cannot be dissuaded from this notion. Attempts to disprove it will only confirm it to your altered perception.