What is a spring ?
A spring is a constraint similar to a joint , the only difference being that , in a spring constraint the constraint between 2 rigidbodies can be interchanged dynamically.
As in , during program execution one can change the constraint between rbA<->rbB to say rbA<->rbC .
Apart from this and the [...]

Before jumping into joints Director/Ageia requires the user to create a constraint Descriptor. A constraint descriptor is a structure that stores the required parameters that are later on used to create Linear or angular Joints.
A constraint descriptor is of the following format (syntax and parameter list) :
ConstraintDesc(cname,rbA,rbB,POCA,POCB,stiffness,Damping)
were
Cname - name for the constraint , has to [...]