  We show that the horizon instability of the extremal Kerr black hole is
associated with a singular branch point in the Green function at the
superradiant bound frequency. We study generic initial data supported away from
the horizon and find an enhanced growth rate due to nonaxisymmetric modes. The
growth is controlled by the conformal weight of each mode. We speculate on
connections to near-extremal black holes and holographic duality.
