The Sling Rings were small two-finger rings imbued with the power to open and sustain dimensional portals between any two points within the universe. They are used by the Masters of the Mystic Arts, and their apprentices, to facilitate long-distance travel where a temporary portal is desired. Sling Rings are typically worn on the left hand only when conjuring a portal, and this is achieved by the wearer waving their right hand in a circular pattern while focusing on a destination beyond the space directly front of them. Most of the time they are simply carried until needed, typically somewhere on a user's belt.
When Doctor Strange was being trained in the Mystic Arts in Kamar-Taj, he was having difficulties mastering the Sling Ring. It wasn't until the Ancient One left him atop Mount Everest that Strange finally mastered the Sling Ring.
After the Avengers reversed the effects of the "Snap", Doctor Strange used his Sling Ring to travel from Titan to Earth while Wong and the other Masters of the Mystic Arts used Sling Rings to open portals from Wakanda, New Asgard, San Francisco, and multiple other locations to bring the Avengers' allies to the ruins of the New Avengers Facility to battle Thanos and his army.
|In Earth-616, the Ring of Zona was a Sling Ring and one of several mystical artifacts that Doctor Strange was able to procure from his Sanctum Sanctorum as he was fleeing the Mindless Ones. He used it to render Spider-Man invisible to the Hobgoblin so as to protect the web-slinger from the magically enhanced supervillain.||Symbiote Spider-Man: Alien Reality #1|