So it's time to explain exactly how the Driver works, so here's a close up of the design.

The Sardior Driver has two dragon-head-shaped slots on it. The heads slide open to reveal the slot where the scales go. In order to activate the driver, Kiriya first must flip the horn switch from being folded down to folded up. The one on Hydra's right is the main Scale Slot, which is where the Amethyst Scale is inserted to instigate his transformation into Kamen Rider Hydra, activated by closing the Dragon Head around the scale. Once activated, the secondary slot can be opened up and a second Gem Scale can be inserted and activated in the same way, changing his pauldron from the base Amethyst Head to any of the other Dragon Head pauldrons.

To cancel transformation, the Horn Switch need simply be folded back down. While most of Kamen Rider Hydra's scales are incapable of instigating a full form change, he does later gain access to the Ruby Scale, one of Sardior's own dragon scales, which grants him the power of all Gem Dragon Scales at once, and gives him a pair of powerful swords that can be linked into a single sojinto, known as the Rublader..

The Sardior Driver is designed by me.