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 possibly "Overlords" as well. It appears that this control is connected to the aliens' radio-wave based communication.
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 in to 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 weened 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."In What Hides Beneath, Anne 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 affect 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.
It is also seen without guidence to a host a harness will attept 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 Love and Other Acts of Courage, the skitter known as Red Eye, through Rick, refers to the harnesses as "chemlochs". This may be the true term for the creatures.
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.
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 Death March its 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.
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.
In Homecoming it was revealed the Overlords do have a method of removing the harness though only uses it when the subject surrenders or is somehow alternately controlled.
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 somone 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 occured in the series.
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 deharnessed the children started seizing before they were touched. This could have also been due to the "guardian" skitter being killed prior to harness removal.