Death Star 10188 Help with step 8 on page 76
The &quot;LEGO Flat Silver Technic Axle Joiner Perpendicular with Extension (53586)&quot; smooth bore piece  is secured to a technic pin on piece &quot;Technic, Pin Connector Perpendicular 3L with 4 Pins (48989)&quot; installed in step 5, with the relevant pin marked in step 7 in my marked up directions below:

The smooth bore piece connects to the pin holding them together, with the lower axle connection preventing rotation.