PSI is the acronym for "Pilot Support Interface" and describes the heart of the Chimera mobile weapons platform. It shares some characteristics with the ProtoMech cockpit from the Clans, but it's not as deadly as its Clan counterpart.
The PSI module consists of two parts: the "cocoon" is a tiny cockpit in which the Chimera pilot sits like on a chair with armrests. The pilot controls the limbs and the body of the unit with five medical computers that gather information from a number of implants in the pilot's muscles. While the system has been used for centuries to operate artificial limbs it is almost unique for a combat unit, since it's exactly the same way the EI implants help a clan pilot operate a ProtoMech. But there is a huge difference: the PSI module also boasts a compact gyroscope, which keeps the unit on it's feet unles the pilot tells the system to move. On a ProtoMech, the EI implant allows the pilot to use his own sense of balance, making a gyro unnecessary. However, this has a SERIOUS drawback: the EI implant requires a very agressive surgery, and it puts the wearer's nervous system under an enormous stress that rapidly affects the entire body. Anyone who gets an EI implant has less than 5 years to live before the implants cause catastrophic damage to the nervous system and result in a fatal aneurism.
The implants used for the PSI module are different. They do not provide any feedback to the pilot's nerves. In fact, the only thing the implants do is sending a constant "jamming signal" to the wearer's muscles, causing them to relax even if the pilot attempts to move. That way, he may sit quietly and comfortably in his chair while moving the machine around him like it was his own body, and use the enhanced sensors like a sixth (or even seventh) sense for maneuvering and attacking his foes.