  We discuss the origin of the low-energy cutoff, or LEC, seen in the radio
spectra of many extragalactic jets and relate this to the spin of the
supermassive black holes that presumably power them. Pion decay via
proton-proton collisions is a possible mechanism to supply a secondary positron
population with a low energy limit. We expect that pion production would occur
in advection dominated accretion flows or ADAFs. In radiatively inefficient
ADAFs the heat energy of the accreting gas is unable to radiate in less than
the accretion time and the particle temperature could be high enough so that
thermal protons can yield such pion production. Strong starbursts are another
option for the injection of a truncated particle population into the jet. The
role of both mechanisms is discussed with respect to the black hole spin
estimate. The energy demanded to produce the pion decay process involves a
minimum threshold for kinetic energy of the interacting protons. Therefore the
mean proton speed in the flow can determine whether a LEC is generated. In
ADAFs the random velocity of the protons can exceed the minimum speed limit of
pion production around the jet launching region in the innermost part of the
flow. Finally we summarize the additional work needed to put the model
assumptions on a more rigorous basis.
