The Mask brush paints a black mask on our mesh, locking the masked polygons in place. When we try to use our other brushes, the masked polygons will not be affected. Masking out the silhouette of new features (like the nose on a face or the arm socket for an arm), inverting the mask, then extruding the new feature with Snake Hook is a fast way to create forms that can then be refined further with other brushes:
The Mask brush prevents the masked polygons from being affected by other brushes
There are also the Box and Lasso mask brushes that have the same effect, but they let us create masks by dragging a box selection or drawing ...
