The slightly hourglass-shaped, rubber-bristle brush of this mascara has bristles that work like miniature teeth, "grabbing" each lash to elongate it. The brush also allows you to deposit extra mascara at the base of the lashes for more dramatic, eyeliner-like impact.
Definitely best for length rather than thickness, you can achieve surprisingly long lashes with only minor clumping along the way. You may notice some minor smearing until this sets, but it's easily cleaned with a dry cotton swab. On balance, this is a good mascara, but it's worth mentioning that there are equally good options at the drugstore.