Hi,
It's in de dynamics tag of your spline dynamics.
Go to the properties tab and set the 'rest mix' to 0% and your spline will follow your Null correctly.
If you set it to be a 100% your spline won't follow your Null at all, if you set it to be a 50% it your spline will follow the Null at half the speed and so on.
Hope this helps,
-W-