The Arcle (アークル) is Kuuga’s transformation belt. An ancient belt created by the Linto Tribe and used by the original Kuuga, Riku. While being attacked by Zu-Gumun-Ba, Yusuke grabbed the belt and after putting it on, it immediately fused into his body. When he transforms, it reappears around his waist and is always the same color as his current form. A stone embedded in the belt, called Amadam (アマダム Amadamu), is the source of Kuuga’s power and able to extend the lifespan of a person.
Full details on this CSM release of the Arcle is available in this Toku Toy Store News article.