With this command, you can create a new spline segment. Select one of the points that you want to separate. After using Break Segment, you will have a new segment and all points on either side of the separated segment will become a new segment. If the selected points are not consecutive, a number of spline segments will be created, one for each of any consecutive points and one from the remaining points.
To add a new segment to an existing spline, you must first create the first point of the new segment. Before the break, it is still connected to the old segment. If you now use Break Segment then the new segment will start here; you can now add new points using the Add Points command.