what is the simplest way to understand the physics of relativistic jets? we know that they have axial symmetry with very tight angular spread, presumably aligned with the axis of rotation of the black hole. It seems to be a mechanism not entirely related with mass accretion because that tends to be constrained in the rotation plane
Firstly, relativistic jets are not just produced by black hole accretion, but from a number of different astrophysical phenomina and objects such as pulsars, Magnetars and Magnetar-energized supernova explosions, collapsars etc.
The main source of power of black holes/AGN and GRB jets is the rotational energy of the central black hole and/or its accretion disk. The naturally occurring low mass density and hence high magnetization of black-hole magnetospheres suggests that the relativistic jets originate directly from the black-hole ergosphere as Poynting-dominated outflows. The dynamicas of these are too complex to explain in detail here but see some papers from S. S. Komissarov for more details on these mechnisms. A slightly easier formation to explain/make easier to explain, is that of jets from pulsars...
[simplified version] The suspected primary cause of the formation of relativistic jets for pulsars and magnitars is magnetic hoop stresses which cause the bulk flow away from these objects, to be collomated and redirected from the equitorial disc back towards the polar axis, this causes large increases in the total pressure in the polar region (outside of the termination shock) and causes vast accelleration along the rotation axis and away from the star (note, there is much more going on but this is the general gist!). The formed jets remain collimated (cyclindrical - with perhaps some sausage instabilities as it travels away from the star) due to the 'local' Z-pinch in the 'vacinity' of the star (see here for an awesome animation of such a flow). A simulation of the Crab nebula showing the synchrotron emmission from such jets is below
these simulations tell us that a mechanism like that discribed is at work, because when we compare them to the real images we find a remarkable resemblance
There is much more information avalible on dynamics or relativistic jets and their formation. I am sure with the information above and Google you will be able to make further progress...
I hope this helps.
Ps. I have found some of my simulations, they are cool to watch and provide an insight in to what I describe above.