Here is the final picture that show the two key frames that make the downward movement and notice the BLACK line between frames 1 to 20 show what the BASE layer will do and the RED line (also BLACK) shows what the new layer will add to the movment. At the end of the new layer the BASE layer will take over and continue with the BLACK line.

The most important thing to remember is that the Spline tool will go through all the points on the line. So if the keys are close together and are not evenly spaced or a have a wide range of movement it will cause the spline to swing in large manner. It is always the best thing to do animations is to use the graph editor so you can see the changes and where the over-shoots are and try to correct before going to far down the editing path.

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13