Here's what Tama say:
The HH915D features the same long footboard as our Speed Cobra Bass Drum Pedal. Its core design is based on the Iron Cobra Hi-Hat Stand, widely recognized as one of the best performing hi-hat stands in the world. The longer footboard offers footwork flexibility, added control and sensitivity. The latest innovation on this stand, the new "Quick-Set Clutch", allows hi-hat cymbals to be quickly attached and removed. The Hinge Guard Block on the footboard holds the bearing more evenly, minimizing stress and maximizing smoothness.