  There is a general consensus that magnetic fields, accretion disks, and
rotating black holes are instrumental in the generation of the most powerful
sources of energy in the known universe. Nonetheless, because magnetized
accretion onto rotating black holes involves both the complications of
nonlinear magnetohydrodynamics that currently cannot fully be treated
numerically, and uncertainties about the origin of magnetic fields that at
present are part of the input, the space of possible solutions remains less
constrained. Consequently, the literature still bears witness to the
proliferation of rather different black hole engine models. But the accumulated
wealth of observational data is now sufficient to meaningfully distinguish
between them. It is in this light that this critical paper compares the recent
retrograde framework with standard "spin paradigm" prograde models.
