Worn by its wielder as a forearm armor, Yiang Sjing's shield (Y.S.S.) seems like a decorative piece of armor.  

When its wearer is successfully hit by a weapon attack, Y.S.S. deploys itself to protect him, giving his owner a +2 AC, without requiring his owner to have a free hand.  If the wearer's new AC is higher than the attack roll made against him, Y.S.S. fully absorbs the impact and stays in its shield form until a) all its HP are gone or b) its wearer commands to fold back.

Y.S.S has 10 HP. At 0 HP, it gets back in its forearm armor form until a long rest.

Shields require the Utilize action to Don or Doff. You gain the Armor Class benefit of a Shield only if you have training with it.

Notes: Special, Utility, Warding