Refinement: Dial it in
By the refinement phase we've got the basic configuration for the product and we've done enough development to start focusing in on the details of the design.
A good example is the Nektar drug delivery device. It uses a hand operated air pump to create air pressure for the device. One of the challenges was to design a handle that was comfortable for a large hand as well as a small hand that could effectively be used by someone with weak hands. We created the model shown above where we could attach different handle parts to try different designs before going to production.