While equipped, it multiplicatively increases damage done to enemies under any Crowd Control effect, applied by either the gem's owner, traps or other players. An attack that applies such effects is not amplified if the target is not under CC yet. Upgrading increases the damage bonus.
At rank 25, it also gives a permanent aura of minor Slow, effectively making the gem always work against all enemies within 15 yards. This works to make the gem a useful and reliable tool to almost any Nephalem hero in almost all cases.
Bane of the Trapped