Are you sure he didn't mean split?
If he meant split, you have too few vertices and you need to make some more or split the ones you have. You can do this by highlighting a track segment, either right-clicking or in the 'Segment' menu find Split-Up, and click it. A vertex will be added in the center of the segment. This will allow you to make smoother transitions. BE ADVISED that too many vertices are also a problem because they are more noticeable and you do not want to be able to tell where a vertex is while in the simulator.
If he truly did mean slip, then I have no idea. [;)]
[dorkhat]