Source Advanced Player’s Guide
Thin leather flaps keep the hundreds of tiny, fishhook-like needles dotting the surface of this black vest from harming you while you wear it. However, any creature that injures you with a natural or unarmed attack must make a DC 15 Reflex save or take 1 point of damage. Any creature swallows you it takes 1 point of damage each round until it either spits you up, you escape, or you die (at which point the vest has sustained enough damage to no longer serve as a threat). The vest can only be worn over light armor or no armor.