Collins, Frank, Bjorkman, & Livio (1999) took this standard model one step forward by proposing that the asymmetry in the RSG wind is the result of rapid rotation of the spun-up RSG progenitor. This model provided the important addition to the GISW model of allowing for angular dependence in the wind velocity (see also Dwarkadas, Chevalier & Blondin 1996): A slower RSG wind at the equator makes it easier to match the observed ring expansion of 10 km/s.