This gem causes Critical Hits to apply a Bleed effect to the enemy. Each Critical Hit, regardless of Proc Coefficient, applies one, but they do not stack, only being renewed to full duration. The effect can be applied by your pets/minions and follower. Upgrading increases total damage done (and damage per tick).
At rank 25, it also increases Attack Speed for every bleeding enemy close to the player. This includes enemies that are under any Bleed effect (not just the Pain Enhancer's own Bleed) applied by that player, pets/minions, and follower. If an enemy bleeds from more than one source, it will still only grant one stack of Blood Frenzy.
This gem is of most use to Physical skill builds. The effect itself has a Proc Coefficient of zero, and cannot score a Critical Hit (increasing flat damage proportionally to the Critical Hit chance and damage instead).
Xiangyi, most beautiful of Xiansai master gem cutters, crafted this gem in honor of a former lover. The angrier she got, the faster she worked, and the gem was ready in no time. Only after she had tested its efficacy was she finally satisfied.