The harness was designed at Big MT's Y-17 medical facility with autonomous casualty recovery in mind. The suit tracks the vital functions of the user and, once a certain injury threshold is reached, it takes over the motor functions of the user and walks them to the designated home base. It is a miracle of research, employing intelligent fibers and artificial muscle, rather than the crude hydraulics that dominated military technology back in 2077.
However, due to the prototypical status of these suits, they suffer from several major issues. First, if a home base is not defined, the suits enter an unpredictable wander state. Second, while it is possible to calibrate the upper injury threshold for the override to kick in, the sensors cannot detect if the user is still alive. Third, the suits record all user movements and can effectively "learn" to replicate their shooting and fighting reflexes, but lack any IFF software. All of these issues combined have led to them breaking out of the Big MT's research facilities and wreaking havoc around the crater.
With no defined home base and no ability to detect the death of the users, the suits acted as mobile coffins: soldiers and researchers trapped inside had no ability to shut them down and were forced to suffer a slow, agonizing death. Even after two centuries, the suits, still carrying the bones of their original users, continue to function, scouring the Big MT of undesirables.
The master trauma harness worn by a Mr. Harris, a researcher of the X-12 research center, malfunctioned before the war as he choked on a buffalo gourd seed. The harness began attacking other employees, who were forced to seek assistance from X-8 researchers. However, while the researchers tried to subdue the berserk harness, the person carrying the K9000 slipped and dropped it. Unarmed, the team had to flee, but not before sealing the X-12 research center with a forcefield. For two centuries, the harness was kept beneath the forcefield, armed with the K9000.
With the exception of the Y-17 master trauma harness, Y-17 trauma harnesses exclusively use higher-end energy weapons.
The harness has a very similar appearance to the space suit, but without gloves. Sometimes, it even has a helmet.
Despite the lack of gloves, the suit is somehow able to manipulate the hands of its dead occupant and use them to operate weapons. Head shots do extra damage like with most living creatures, but the head is only a dried skull.
One of the audio logs for the Y-17 master trauma harness is a reference to Doctor Who episodes 'Silence in the Library'/'Forest of the Dead'. In these episodes, the protagonists are chased after by creatures which possess space suits after cannibalizing their inhabitants. The last words of the suit's inhabitants loop endlessly. The last audio statement of one such suit, and the line heard in Fallout, is 'Hey, who turned out the lights?'