Eva's spine was something that we hadn't tried before. An FK spine was requested by the client but the complaint was FK had to many controls.
Adding just a control at the top and bottom of an FK system and having the mid joints controled with a fixed blend to the top and bottom controls doesn't allow for the sort of flexability that is needed.
The solution was to add a third control that didn't directly bend the spine but instead changed the blend ratio from top to bottom. This allowed the animator to decide where the flexing of the spine would occure.