There is still no good explanation of why only some AGN produce strong jets. One of the most impressive examples to date is the observations of NGC1275 by Venturi et al. In this source, at least, there is an indication that jet collimation is not achieved close to the event horizon of the putative black hole, but over length scales some 100-1000 times larger. Despite this, it is reasonable to suspect that the association is genuinely physical. The most significant type of connection is that jets are generically responsible for extracting both angular momentum and energy liberated by gas as it flows radially inward through the disk.
Magnetic field provides a plausible link between disks and jets.
Magnetic jet collimation We have already, indirectly, referred to some of the evidence that magnetic field plays a large role in jet collimation. In powerful AGN jets and YSO outflows, neither gas pressure nor radiation pressure appears to be adequate to the task of collimating large momentum flux. This leaves magnetic field as the prime candidate. Ultimately, this must be gas pressure. Morphologically these most resemble magnetic flux tubes that have been activated, somewhat similar to H and X-ray images of solar prominences.