The H.A.L.O. System was used to control the probability constant of the Chrono Strings. Through this system, the probability of gaining energy from Chrono Strings could be directly interfered with, allowing a much larger amount of energy to be drawn out. It utilized a quantum computer to scan the minds of the Angels to allow them to accomplish this. Appearance-wise, it took the form of a white halo above the pilot's head.
However, if the scan level was too high, it could cause the Angel to go into a trance state, where their body would enter a dangerously unstable condition. Also, if the flow of the scan was reversed, the system would backfire into the pilot's mind. If this happened for only an instant, the Angel would be forced into a comatose like state, though nothing not treatable by a UPW medical facility. Any longer exposure to a reversed scan would likely have irreparable consequences to both the Angel's mind and body.
In TCY 417, Noa developed an AI system that adapted the H.A.L.O. System so that the probability controlling aspect no longer depended on the pilot, and was rather handled by an AI. This system was used in the mass production Emblem Frames, allowing them to be piloted by anyone.