How Do Fractional Bond Orders Arise in Resonant Molecular Structures?

[mahela007]

What is resonance when it comes to molecular structure? I understand that the concept of resonance arises when there is more than on possible lewis structure. However, everything I read says the atoms don't oscillate between structures and that in fact, they assume a constant shape.
My question in, how do bonds of intermediate length form between these atoms? for example bonds with a bond order of 1.33?

 

[Ygggdrasil]

Resonance comes from the fact that although we think of bonds as the sharing of electrons between two atoms, some chemical bonds are formed from the sharing of electrons between three or more atoms. In this case, the molecular orbital(s) representing the bond do span the three or more atoms and the (average) number of electrons between two atoms is indeed the intermediate number of electrons predicted by the resonance hybrid.