In the words of Silktone:

The Silktone Fuzz has been updated with some cool new features starting with a couple of tone controls to subtly shape the sound of the fuzz. The tone controls are a product of the cleanup circuit so they shape the tone before heading into the fuzz. The bass shape goes from a tight and forward sound when turned down to full and round when turned up. The treble shape subtly rolls off the high end for a softer fuzz tone. The shaping is much more apparent at cleaner settings too.