The Affinity Twin is a technology painstakingly developed by the Cyclan which allows one being’s mind to take total control of another being’s body.  It is a symbiotic artificial life form containing two components, one of which must be injected into the dominant being, the other of which must be injected (more or less simultaneously) into the subject being.  The dominant being’s mind does not actually occupy the subject being’s brain, although it does have limited access to that brain’s memories, reflexes, instincts and learned behaviors.  It is more akin to a psychic connection between the two, re-routing the subject being’s senses to the dominant being’s brain, and the dominant being’s will to the subject being’s actions.  The dominant being’s body is effectively comatose while the link is in effect.  The connection is instantaneous, even over interstellar distances (similar, and probably related, to the Homochon technology enabling the Cyclan’s gestalt consciousness).  The connection is not broken until one of the bodies dies, at which time the surviving being’s consciousness regains control of its own body. 

The Affinity Twin is composed of fifteen biological units which must be assembled in a precise sequence.  That secret was stolen from the Cyclan’s laboratories and all records of the correct sequence and its development were destroyed or falsified.  To recreate the correct sequence by trial and error could take centuries.  This technology would give the Cyclan the ability to place operatives at the very top of governments they would otherwise find difficult or inconvenient to manipulate in their conventional fashion.  It would greatly accelerate their conquest of the galaxy, and recovering the secret is their top priority.  The secret is given to Dumarest in Book 4, Kalin, although he doesn’t realize this until Book 6, Lallia and doesn’t actually put it to use until Book 8, Veruchia.  Dumarest realizes using the formula to synthesize a working dose of the affinity twin is risky (doing so requires advanced biotech facilities, and will reveal his whereabouts to the Cyclan if they discover it has been done), and regards its use as loathsome, so he uses it only sparingly throughout the rest of the series.