A Synthetic brow brush shaped to groom + carve out the brows.
Dense enough to define under the brow but thin enough to mimic tiny brow hairs. Great with powder, gel, or cream brow products. 
 
I like to use a brow gel to groom the brows into postion using the spooly. Then dip into your chosen brow product, carve out the underneath of the brow. focusing the depth on the tail end. Flipping the brush on its side i then like to finely add brow hairs to fill in the sparse gaps.