The concave arches in the sides of the ski, designed to enhance the ski's turning performance. In general, the more sidecut a ski has (up to a point), the easier it will be to turn. Keep in mind, however, that sidecut can have a negative effect on your forward glide and how well your skis track in a straight line. For this reason, pronounced sidecuts are not recommended for in-track striding or for casual touring across flat or moderate terrain.