The Ettin System is relative simple, it is a second head. The second head contains an artificial brain with it's own loaded persona program, and skill nets. This second head is grafted and linked into the central nervous system of the host, allowing the host to access and use the skills of the cyber-head.
Appearances of Ettin systems vary from user to user. Some have the exterior of the head grown to match their own, or to look like a specific person. It is also common for ettins to have fascias and add cybernetic senses to the second head's abilities.
Ettin systems are rare and monstrous cybernetic augmentations, and grafting a second personality into a single body is fraught with problems. Ettin cyborgs have personality disorders, struggle with their own self image, and constantly have fights over which head gets to control the body.
Ettin's are seldom made willingly. The most common course of action is that the second head is grafted to the body of an unwilling person of importance, where it acts as a 'political' officer,supervising the host. They are also used as a form of punishment, or used to force the host to do a desired set of actions. This can be used to make a scientist do undesirable or highly questionable work. Make a soldier carry out unpleasant tasks, and so forth. But usually, it's done as a sort of punishment for the amusement of others.
Self inflicted Ettins are typically looking to expand their abilities, a rogue scientist who needs more ability to delegate, or to use a cyber-head for whatever tasks they want it to automate. Cyberheads are also favored by the most extreme riggers and drone controllers, where the second head acts as a copilot or electronics operator.
The Ettin system has a dark and scattered history, and there isn't a single point of origin for an established system of attaching second heads onto a host. Instead, there are dozens of threads that come together to create a bloody tapestry of cybernetic crimes against humanity. There are instances of critically wounded people having their heads removed and grafted onto someone else to keep them alive. Other stories of mad doctors committing heinous cases of 'to see if we could' and pursuing head and brain transplant techniques. Some simply appeared, screaming and mad about horrible things done in places that can never be found again.
Ettins are victims, and the second head's purpose determines the nature of the hook.
Political Officer Head - the head has a cyber brain and is programmed with the mind of a political commissar, and is obsessed with making sure the host does what they are told, and support the party/cause/dictator. Removing the head will cause it to explode with lethal force. The host has been commanded to carry out an assassination attempt, or was a dissident and is now silenced with shame.
Mutant Head - the head was removed from the body of 'something else' that wasn't entirely human, and it is slowly extending it's control to take over more and more of the body. The host is a true victim and will eventually end up a vestigial parasite on their own body.
Combat Head - the cyber brain in the ettin is designed for combat coordination, and allows for a single person to sit in a combat machine and carry out the role as both pilot and observer.