Editing a Spline
Spline control points respond as you would expect to the translation and rotation tools, but the scale tool brings up a different UI:
These are the spline tangents. If a segment is selected, it shows the tangents at each end of the segment, and if a control point is selected, it shows the tangents of all connected segments.
You can adjust the scale of the tangent by dragging the end of it, which alters how curved the segment of spline is.