The ahn-woon is an ancient Vulcan weapon, a leather strap used to choke or bind an opponent, and the second weapon of the koon-ut-kal-if-fee.
Described as one of Vulcan's oldest weapons, the ahn-woon survives mainly through ritual. After the first round of the duel between Spock and Captain Kirk in 2267, fought with lirpas, the combatants switched to the ahn-woon.
Spock used the strap to strangle Kirk, who appeared to die. In fact McCoy had injected him with a drug that simulated death, ending the challenge and allowing both men to leave Vulcan alive.