  The mechanisms that produce and power relativistic jets are fundamental open
questions in black hole (BH) astrophysics. In order to constrain these
mechanisms, we analyze the energy efficiency of jet production 'eta' based on
archival Chandra observations of 27 nearby, low-luminosity active galactic
nuclei. We obtain 'eta' as the ratio of the jet power, inferred from the
energetics of jet powered X-ray emitting cavities, to the BH mass accretion
rate Mdot_BH. The standard assumption in estimating Mdot_BH is that all the gas
from the Bondi radius 'r_B' makes it down to the BH. It is now clear, however,
that only a small fraction of the gas reaches the hole. To account for this
effect, we use the standard disk mass-loss scaling, Mdot(r) \propto (r/r_B)^s
Mdot_Bondi. This leads to much lower values of Mdot_BH and higher values of
'eta' than in previous studies. If hot accretion flows are characterized by
0.5<s<0.6 -- on the lower end of recent theoretical and observational studies
-- then dynamically-important magnetic fields near rapidly spinning BHs are
necessary to account for the high eta~100-300 per cent in the sample. Moreover,
values of s>0.6 are essentially ruled out, or there would be insufficient
energy to power the jets. We discuss the implications of our results for the
distribution of massive BH spins and the possible impact of a significant extra
cold gas supply on our estimates.
