After the initial attacks on earth, Skitters began to abduct children and place what humans refer to as a 'harness' to each of their backs. Appearing bio mechanical in nature, the harness bonds to the child's spinal cord, from which point they can be controlled by Skitters, and the Espheni as well. It appears that this control is connected to the aliens' radio-wave based communication.
They grant superhuman strength and senses, but also give a high dose of an unknown opiate, and modify the hosts DNA to turn them into something similar to skitters.
In Young Bloods it is shown how the Harnesses are attached to its host.
Unattached harnesses resemble large slug-like organisms capable of movement on their own. They are normally stored in a fluid-filled tank. A host child is restrained nearby while the Harness is sent down a chute and is guided onto the child by a Skitter and attached. Once in position the head of the harness reveals a "mouth" like structure full of sharp teeth that clamps down into the victims neck.
Once this occurs the host appears to lose free will and the tail of the harness then clamps down onto the back and the body hardens growing a shell like structure of a complete harness. After the harness has hardened, it appears much less organic and more like technology, so the resistance was not aware at first that they are a form of bio-mechanical creature. After a harness has hardened into place, it does not appear that it can revert back into an independently mobile form, and harnesses cut off children by the resistance are inert.
It is also seen without guidance to a host a harness will attempt to latch on to any living creature they can find; one bit Weaver on the leg when he got to close. They are also strong despite their helpless appearence as Tom had to wrestle one of the harness creature that was crawling up Matt's back away before shooting it.
In Homecoming it was shown that a Harness can inject a parasite of some kind through biting a victim. The pathogen moves through the the victim's capillaries near the skin and lower the victim's body temperature.
Given that the Skitters themselves were mutated into their current form using the harnesses, the origin of the harnesses themselves is unclear. It is possible that they were once free-living organisms before the Overlords modified them, but if this is the case they have been so heavily modified that they are now bio-mechanical constructs which seem far more machine than animal (in their final, immobile state). The other possibility is that the Overlords bio-engineered them from scratch.
According to Michael Harris's theory, when the mechanism's needles penetrate the child's spine "they soften and grow roots to become part of the host's nervous system". The device releases a form of opiate, unfamiliar to humans, which may play some part in ensuring submission to the aliens' orders. It appears highly addictive, as when removed from a child they go into shock, and die from apparent withdrawal. Another element of Harris's theory is that having the child on an IV drip with a man-made opiate allows for removal of the harness, though the process is still not entirely without risk. The removal of Rick's harness proved this theory true, and subsequently he was weaned off of the drugs and survived the procedure. This allows the process to be carried out on other children, including Tom Mason's son Ben.
When attached to a child, skitters are able to communicate and control the subject, manipulating them at will. It is unclear whether ability this relates to the harness, the opiate it releases, abilities of either the Skitters or "Overlords", or some combination of these elements. What is clear though is that this connection remains even once the harness is removed, although with limited effectiveness. This was shown at the end of Prisoner of War when the newly captured Skitter and Rick awoke at the same time. Throughout the rest of the season, Ben and Rick, who wore their harnesses longer than the other children, continued to show signs of being affected.
The effects of wearing a harness are minor at first; in addition to loss of will, harnessed children have apparently improved health and vitality, as Rick was cured of his cystic fibrosis, and Ben was shown to be capable of doing 102 pushups, or jump-roping continuously for over three hours. Ben also indicates that while harnessed, time is experienced differently to how humans normally perceive it, and that there's a feeling "like someone's thinking with you." There is also physical changes such as rough, discolored skin.
In "What Hides Beneath", Anne Glass and Lourdes dissect the Skitter prisoner, which had been killed a few episodes prior. They discover what appears to be a harness fused to the Skitter's spinal cord, which may explain their method of communication and ability to control harnessed children. The same episode was the first to introduce the "Overlords", and indicated that they controlled the Skitters in a way similar to how Skitters controlled harnessed children. In addition, it is suggested that the harness actually modifies the host, suggesting the Skitters were mutated from another species that may have been previously conquered by the "Overlords" in the same way they are attempted to with Humans.
It is possible that removing the harness slows the process, as both Rick and Ben were far less developed in their transformation than Megan, despite indication they were harnessed far sooner than her. Also, the Skitter known as "Red Eye," can even control children who have been released from their harness, as seen in "Compass", when Ben was about to kill him, the spines on his back started to glow, and Ben stopped. Also at the end of Compass, Red Eye found Ben and subdued him, causing his spines to glow again. What was being communicated, though, is still unknown. It is also unknown if other Skitters can do this.
Of interest is that despite the continuous effect of the harness on its host, and the feeling of shared thoughts, the connection between Skitter and child is only activated when a direct command is to be issued. This allowed Hal Mason to infiltrate a Skitter base and rescue Ben in Silent Kill.
In Love and Other Acts of Courage, the skitter known as Red Eye, through Rick, refers to the harnesses as "chemlocks". This may be the true term for the creatures.
In Molon Labe it is seen that despite losing free will harnessed individuals can have a level of authority among the aliens as it seems Karen Nadler was leading an entire squadron of Mechs, Skitters, and implied to be Airships. It appears that this authority is due to her connections with the overlord.
In Collateral Damage a group of extremely mutated harnessed children where seen guarding the Nuclear plant. Communicating in skitter screeches and faster than recently harnessed children they were covered in molted skin with bulging heads taking on a Skiter shape. Close inspection of one dead child revealed he was starting to sprout extra limbs.
Humanity came up with a low tech method that releases the victim from the harness. Initial attempts at simply pulling the harness out were unsuccessful and killed the victim. It was later discovered that the harness pumped an unknown substance into the victim and forcibly removing the harness sent the child into shock and led to death from withdrawal. Since Morphine was the closest Earth bound substance it was decided it should be used as a bridge during removal. The harnessed individual is then fed a drip of Morphine before and during the procedure.
Instead of pulling the harness out several people lift the harness exposing as much needle as possible while someone else cuts the needles with a blow torch; cutting the harness off the victim. This procedure is the most successful with only one fatality having occurred in the series. This procedure leaves behind the spikes and internal nucleus of the harness. These allow the former host certain benefits, such as no longer having asthma, needing glasses. It also increases agility, eye sight, and allows for communication with Skitters and Espheni. For the children who have their spikes left behind, it is still unclear as to what the long term effects are. It does put stress on the host’s skeletal muscles, and they are suspected to interrupt stem-cell production, which could lead to shortening the former hosts life-span. (At All Costs)
In "Homecoming" the Overlords claim they have a method of removing the harness, in order to coax Tom Mason onto their ship. The offer, at least, was later revealed to be a lie, though it is still unclear if the Overlords are capable of removing the harnesses themselves.
It should be noted that while no longer mobile the harness creatures seem to be aware of what the humans were attempting as, when Ben's group were being de-harnessed, the children started seizing before they were touched. This could have also been due to the "guardian" skitter being killed prior to harness removal.
In "Death March", it is shown that if a harness is hit with enough blunt force (in the episodes case a moving vehicle) the harness will die but the child it is attached to will survive, freeing him or her from the harness' control but will still suffer from side effects.
With the arrival of the Volm, humanity was given a machine capable of removing Harnesses safely with no risk to the host. It does however cause the Harness pain, as shown when Anne Glass was removing Diego's, his harness could be heard screeching. Anne Glass noted that, with the machine, she was able to remove 25 harnesses with no children dying. (On Thin Ice)
Humanity later discovers safely removing the harness externally leaves it's internal nucleus intact. Fibers extending out to their spikes are being regenerated by the nucleus. The Volm de-harnessing machine allows someone to extract the fibers, and uproot the nucleus without affecting the host’s spinal cord or nervous system. However, all the benefits that it provides will be lost with it, i.e. Ben needing his inhaler again. (At All Costs)