Stitch
Click for movie example
To insert a new spline into existing geometry, called “stitching”, click the Add button and click on the splines where you want the new control points inserted. To maintain the existing splines’ curvature, keep the <Shift> key on the keyboard pressed while clicking.
USERS OF PREVIOUS VERSIONS TAKE NOTE:
The Add accelerator key (normally <A>) is now defaults to AddLock (since AddLock will do almost everything Add will, and more). <Shift><Add> now defaults to a traditional Add.
To encourage new users to model with AddLock, the original Add, Insert, and Group (because its not needed often) buttons have been removed from the default Modeling toolbar. (The functions still exist on the accelerator keys, and you can add them back to the toolbar if you want to.)