The complete steering column is changed. Part of the steering column is cut out and removed, the EZ PS part more or less is fitted in between the shaft ends. The outer shaft is also changed, it has to be mounted rigidly to the body as it has to respond to the torques.
When you turn the steering wheel, the ECU "sees" the input torque from the top part of the column and compares it to the lower part of the column. If the torque level goes over a limit, electric assistance is put to the lower part of the column.
There are two methods when you want to install a EPS. Either buy a complete column from EZ or another supplier, this will fit in place of the original steering column. The other method is to buy a kit. Then you cut your existing shaft, remove a length and weld the EZ (or other supplier) part to the old shaft. The kit option is only doable if you know what you are doing and have a good level of welding. You do not want to end up with an unsafe steering system.
Cheers, Dion
(have adapted about 30 cars with EPS. Not from EZ though)