As the cars rotate on the same plane as the coupler centreline and one coupler is in effect a round shaft, this allows the 'fixed' coupler to rotate through 170 odd degrees with the car as the rotary coupler does the rotating.
It's a bit like a door hinge, one side is fixed, the other moves.
Also the hose bags, or air lines are normally slightly longer to enable the rotating action. Well the Pilbara has longer hoses anyway.
I may have some snaps at hand.
Okay, here's a pic:
The rotary coupling is on the left, with the fixed or square on the right.