AutoMask FiberMesh

From ZBrush Info

The Brush >> Auto Masking >> AutoMask FiberMesh button allows your brush to control a specific segment of each fiber that will be affected by the brush as you sculpt or paint. The curve below this button allows you to control how the Auto auto-Mask will be applied to the individual fibers.

The Auto Masking menu in the Brush palette

Bottom to top represents Mask intensity. Left to right is the span of the fiber from root (at left) to tip (at right). There is no end to the control you have by using this curve with Groom Brushes to work with your fibers.

Note: Auto Masking does not actually paint a Mask on your model. Instead, it treats the mesh within your brush radius as if it has already been Masked based upon the Auto Masking settings.