  Relativistic jets are launched from black hole (BH) X-ray binaries and active
galactic nuclei when the disk accretion rate is below a certain limit (i.e.,
when the ratio of the accretion rate to the Eddingtion accretion rate,
$\dot{m}$, is below about 0.01) but quenched when above. We propose a new
paradigm to explain this observed coupling between the jet and the accretion
disk by investigating the extraction of the rotational energy of a BH when it
is surrounded by different types of accretion disk. At low accretion rates
(e.g., when $\dot{m}\lesssim0.1$), the accretion near the event horizon is
quasi-spherical. The accreting plasmas fall onto the event horizon in a wide
range of latitudes, breaking down the force-free approximation near the
horizon. To incorporate the plasma inertia effect, we consider the
magnetohydrodynamical (MHD) extraction of the rotational energy from BHs by the
accreting MHD fluid, as described by the MHD Penrose process. It is found that
the energy extraction operates, and hence a relativistic jet is launched,
preferentially when the accretion disk consists of an outer Shakura-Sunyaev
disk (SSD) and an inner advection-dominated accretion flow. When the entire
accretion disk type changes into an SSD, the jet is quenched because the
plasmas brings more rest-mass energy than what is extracted from the hole
electromagnetically to stop the extraction. Several other observed BH disk-jet
couplings, such as why the radio luminosity increases with increasing X-ray
luminosity until the radio emission drops, are also explained.
