Spring-Loaded Shield (5e Equipment)
From D&D Wiki
Armor (shield), common
This shield is made up of several parts that fold up into a small bracer, offering no bonus to AC when not in use. While it is in this form, you can transform and don the shield as a bonus action using a flick of your wrist. While deployed and donned in this manner, the spring-loaded shield is effectively identical to a mundane shield. After a spring-loaded shield has been deployed, you can fold it back into its normal form as an action.