The Furai Knuckle is Karakuri Ball No. 08 from Ninpu Sentai Hurricaneger. The Karakuri Balls are capsule-style "Variable Attack Units," designed to resemble gashapon capsule toys: when a Shinobi Medal is inserted into one of the Karakuri Giants (Senpujin, Gouraijin or Tenkujin), the robot launches one of these balls from its chest, which splits open to reveal a small weapon mecha inside. The Furai Knuckle belongs to Kabutoraiger and manifests as a powerful fist/leg module.

The Furai Knuckle truly shines in combination: together with Karakuri Ball No. 07, the Furai Head (piloted by Hurricane Red), it forms the Karakuri Warrior Furaimaru. Here the Furai Knuckle becomes Furaimaru's legs, while the Furai Head makes up its upper body and head. Furaimaru is far from a mere sidekick, it is the crucial piece for the big combination: during the transformation into Gourai Senpujin (Senpujin + Gouraijin + Furaimaru), the Furai Head supplies the helmet and the Furai Knuckle forms the mighty fists of the combined robot.

As a DX toy, Bandai Japan released the set on June 19, 2002 as "Karakuri Ball Series 03: Furaimaru," which includes both the Furai Head and the Furai Knuckle. Like the entire Karakuri Ball line, it begins as balls/weapons that mount into the chest pods of DX Senpujin, DX Gouraijin and DX Tenkujin; from there the standalone Furaimaru assembles and can be broken down into the hands and helmet for the towering Gourai Senpujin. The model is mostly plastic, but the feet are solid die-cast. Combined with the separately sold DX Gouraijin and DX Senpujin, it completes the full Gourai Senpujin. The same gear also appeared within the mecha system of the international adaptation, Power Rangers Ninja Storm.