I spin the iced cake on a turntable and use the tip of a small offset spatula to achieve this look.
AUse a small offset spatula and place the cake on a turntable. Gently put pressure on the cake and turn. Keep turning until you reach the top. Follow me?!
I love rustic cakes, this video could give you a visual idea. Sorry if my first post was confusing. The person who made this video is also a member on CC, maybe she could give you some pointers.