The ring emits an electromagnetic wave preventing certain cards from being activated on Duel Disks. The ray is emitted while a part of the ring is rotated a certain way. Ryuga's own Duel Disk was equipped with a shield from the ray.
Ryuga used the ring during his application to become a Duel Academy teacher, in which he needed to defeat 50 students. Among Ryuga's opponents he used the ring on were Syrus Truesdale and Jaden Yuki, preventing both of them from using Spell Cards. However Jaden was able to win the Duel without the use of Spell Cards and the ring got damaged at the end of the Duel.