This brush is perfect if your eyelashes are straight and not very curly. It will cover them from root to tip, give them more body and lift them.
Lea Michele loves Covergirl Clump Crusher Mascara ($7.99 at Ulta Beauty), while Gigi Hadid prefers Maybelline Push up Angel Washable Mascara, the brand for which she is the face ($11.90 at Amazon).
The trick with this brush is to first pass the concave part from root to tip, then add definition to the tips using the convex part.