The Ring of Power is used as part of the Radial Equipment Menu, which is opened by pressing F (by default). It displays any ring equipped onto the player (in red tab) as well as rings in their inventory (in green tab); this ring can be both equipped and 'unquipped' with a left click. This allows for easy management of ring equipping; however, note that only one ring can be applied at a time.
When equipped, it will give an additional 50% boost to that player's damage output. At the same time, it will drain its charge while doing this, and will run out after about half an hour of being active. While active, an additional green flame particle effect is added to the player's melee attacks.
The Ring of Power must be charged when depleted to reuse. This can be done by right-clicking the ring, and it will consume the player's experience to recharge the ring to full power, the amount of experience consumed depending on the amount of the ring's charge that needs to be replenished.