In AutoCAD 2012 multifunctional Grips have been added to many objects!Polylines have extra Grips to make editing much easier. Secondary Grips are located at the midpoint of each segment. Hovering over a Grip provides several valuable editing options such as Stretch, Add or Remove Vertex and Convert to Arc/Line.
Look for these powerful grips in Lines, Arcs, Elliptical Arcs, Dimensions and mLeaders. 3D Faces, Edges and Vertices also have them. Just hover over a Grip to access numerous relevant options.