Animate target object weights for geometry constraints
An interesting technique you can use with geometry constraints is to animate the target object weights specified by the targetObject Wn channels. You can vary the weights from 0 to any value, so that as an animation progresses different target objects can take turns driving a constrained object’s position.
If all the target objects have the same weight (the default), the target point is taken from the first target object you selected when you created the constraint. For instance, if all the weights are 1, the target object whose weight is specified by the constraint’s targetObject W0 channel provides the target point.
You should animate the target weights so that only one target has the highest weight at any given frame.
Note that objects constrained by geometry constraints are motion history dependent.