Maintain Curvature
Normally, during modeling when you insert a new control point into an existing spine, the curve changes shape. To make the existing splines maintain their curvature, keep the <Shift> key on the keyboard pressed while adding the new control point. Beware, for the program to keep a spline’s shape, it must add bias to all the existing control points around the new control point (as many as eight new biases!) Typically, you don’t want to maintain curvature because if there is bias, when the control points animate, the bias will have to be animated also, (a time-consuming process).
Click for movie example
In the above example the first time the new CP is added the <Shift> key is not depressed so the curvature is not maintained. The second time the CP is added the <Shift> key is depressed and the curvature is maintained.