This is a flat brush, also known as a tap brush, from Sephora's Professionnel line. I use it when I want to gather a lot of eyeshadow onto a brush in order to create a more dramatic look.
It is easy to use. I literally tap the brush onto the eyeshadow. I keep tapping until I think I have too much product on the brush, which is actually the right amount. Then, I lightly blow on the brush to get rid of any clumps of eyeshadow. Next, I tap the brush onto my eye lids. (I like to hold the brush upside down while doing this.) I do this about three times in order to build up enough coverage for a smoky eye look. Then, I hold the brush with the bristles up and apply the eyeshadow through the crease and above, with a stippling motion. The result? An easy monochromatic smoky eye.
This is one of those brushes that make you appreciate the value of pairing the right product with the right applicator.