Annular confinement beam

The Annular confinement beam, or ACB for short, is an essential component of the transporter system. Generated by the primary energizing coils, the ACB confines the transporter matter stream when it is on its way to the target destination.

When deflector shields are active, an ACB cannot penetrate the shield envelope, making transport almost impossible, except through the expert use of sensor windows. (TNG: "The Wounded")

The ACB also makes it possible to transport objects and persons at warp, although the velocities of both vessel and target must be identical. (TNG: "The Best of Both Worlds")

In the event of an ACB failure or malfunction, the object in transport will suffer pattern degradation, causing severe injury and even death in the case of living creatures. Commander Sonak was killed in such an incident in 2273.(Star Trek: The Motion Picture)

The ACB can be intentionally breached by the transport subject during the initial stages of transport if their bodies contain certain material compounds. However doing so results in a massive energy discharge as the beam fails, and can be lethal as described above.(TNG: "The Hunted")

In 2368, Miles O'Brien was able to boost the confinement beam in order to beam down one person to the surface of Mab-Bu VI's moon that was ravaged by heavy electrical storms. Geordi La Forge estimated the chances for success at 50:50. (TNG: "Power Play")

The only time that the ACB is disengaged is when a transport in progress is suspected to contain some explosives, or other dangerous material which is not allowed onboard a ship. The targeting scanners are then reset to a point in space, the transporter matter stream is re-routed to that point and the ACB is disengaged, dispersing the material harmlessly into space.

In 2369, Miles O'Brien tried boosting the annular confinement field in order to retrieve a wounded Klingon who was trying to beam aboard DS9. He eventually succeeded. (DS9: "Dramatis Personae")

In 2371, a microscopic singularity explosion in the Sol system caused the USS Defiant's annular confinement beam to fluctuate. (DS9: "Past Tense, Part I")

Annular confinement beams are also believed to be used to allow phaser discharge at warp speeds, but this has not been canonically confirmed.