Signet of Capture

Use Signet of Capture on a boss who has just used a skill you want to learn. Signet of Capture is permanently replaced by that skill. (You can only learn skills of your profession.)

Where To Get It
Magi Magister in Lion's Arch gives you a quest with this skill as a reward. Thereafter, you can purchase additional signets from him or any other Skill Trainer NPC. Alternatively you can purchase it regardless of completed quests at the skill trainer in Quarrel Falls.

Usage
Using a Signet of Capture is the only way to learn elite skills, though regular skills can be captured as well. Signet of Capture must be bought from a skill trainer. This signet can only be used on bosses, and you can only capture skills of your primary or secondary profession.

Signet of Capture will capture the last skill used by the boss, provided you have not already learned it. Once a skill is captured, Signet of Capture is replaced by the captured skill for the remainder of the instance. A new Signet of Capture must be purchased to capture further skills.

Update 6/9/2005
It appears that the Signet of Capture no longer requires the boss to be alive. The boss must be dead before you use the signet and after the signet is cast, you are presented with a menu and may choose whichever skill you do not already have. This makes capturing much easier than it was prior to the latest update.