  We formulate a reduced model to analyze the motion of the core of a twisted
scroll wave. The model is first shown to provide a simple description of the
onset and nonlinear evolution of the helical state appearing in the sproing
bifurcation of scroll waves. It then serves to examine the experimentally
studied case of a medium with spatially-varying excitability. The model shows
the role of sproing in this more complex setting and highlights the differences
between the convective and absolute sproing instabilities.
