Curve Modification

This is an exercise that it is based on the chameleons tongue movement.

- I generate an interpolate curve, defined by 3 points (imitating one way path of the tongue movement).

[IMG]http://img5.imageshack.us/img5/7048/92626779.jpg[/IMG]

- To imitate the movement of the tongue, with a resulting interpolate curve i divide it, connect it with shift (associated with a slider) finally with a resulting interpolate curve (this imitates the movement of the tongue)

[IMG]http://img263.imageshack.us/img263/989/25054249.jpg[/IMG]

- Until there, there is no problem

- My question is: 

How can i do, so the slider associated with Shift, after it reaches it maximun extension (the tongue) and pass that value (of maximum extension) begins to modify the curve? AND begins to way back trough the new curve (that imitates the way back cruve)?

ps: I attached the Rhino and grasshopper archives