The polyline icon allows you to convert a Slice from a traditional orthogonal slice to one that aligns with a curve. To use this feature you must have first added a curve or line to the data.  Select the slice to Align then by clicking the “Align To Curve” button and clicking anywhere along the curve or line will align the view and slice (and optionally the construction plane) to that point on the curve

Note: Curve or line (any Rhino object) must NOT be selected before clicking the “Align To Curve” button.