MOD Spacers™ can be installed in most Duncan® Hardcore yo-yos changing the shape, profile, and response system in minutes. Originally developed for the Pro Z™ yo-yo, MOD Spacers also fit in yo-yos like Freehand, FH Zero, Bumblebee, and more! The off-string version can update your Hayabusa, Flying Panda, Throwmonkey and more to a wider shape & 4 hole response system!