Spin Ring

From Zelda Wiki.org

Quest partway through a password-linked game of Oracle of Seasons, and speak to a Subrosian in Lava Lake who will give you a password. In Oracle of Ages, tell this password to Plen, the Mayor of Lynna City, who will then give you the Spin Ring.

While the ring is worn, every spin attack Link performs will cause him to spin twice instead of once. This effect is useful against bosses that can only be hurt with a spin attack.