After a rotation in field space, the two-field ekpyrotic potential can be viewed as composed of an ekpyrotic direction () and a transverse tachyonic direction (). The ekpyrotic scaling solution corresponds to motion along the ridge of the potential. Perturbations along the direction of the trajectory are adiabatic/curvature perturbations while perturbations transverse to the trajectory are entropy/isocurvature perturbations.
