This brush is carved out perfectly to fit into your lashline to give you that perfect wing liner or defined lash line. 
 
I like to use it with powder eyeshadow to deepen, define and enhance the lash line. Simply dip it into the shadow and stamp it along the lash line. dark purples enhance green eyes, cranberry looks great on blue and any dark brown/black looks great on brown eyes.
 
This is a synthetic eye brush which means it great to use with gel or liquid as well if you want to do a defined wing.