Not counting niche vehicles like 959, Porsche has used 2 basically different AWD systems on versions of the 911 since 1989.
the 911s built from 1989 to 1994 aka 964 used a very complicated version. In this version the trans main shaft is hollow w/ the front drive running concentrically w/ the rear drive output. The rear is the inner shaft and the outer is the front drive.
the front drive connects to a computer controlled clutch pack in the trans nose cone. There are also lateral and longitudinal accelerometers that feed info to the computer to help it decide how much lock to impose. The front drive shaft connects to a front diff, the rear to the usual CWP
The 964 has been described more as a snow oriented system than as a performance oriented system as the the later awd systems are.
The 993 up use a much simpler and lighter AWD setup. They take a regular g50 and
connected the trans main shaft to a viscous coupler in the nose of the trans, the viscous coupler is a purely mechanical system that adjust torque flow though to the front. The 964 system can only be removed after major surgery, The brake hydraulics are shared w/ the awd, and the trans main shaft would need to be welded up. The 993 on system can be unbolted in it's entirety. Yes the reason is for better track performance by being able to steer the car w/ the throttle.
993 viscous coupler
removed
modified and sealed front cover
