  Rapid variability of the TeV emission in several blazars implies a central
black hole mass $M_{BH}<10^8M_\odot$, appreciably smaller than the values
estimated from the $M_{BH}-L_{bulg}$ relation, and Doppler factors for the
$\gamma$-ray emitting fluid much larger than those associated with radio
patterns. We discuss the conditions in the central engine required to account
for the short timescales and large luminosities observed, and propose some
explanations for the inferred kinematics of the source on various scales.
