This must occur before the last few passes over Russia. So the schedule demands a warped sleep schedule. On the day before rendezvous, the crew will use the robot arm to lift the Unity module out of its cradle and mount it pointing upwards atop the airlock, which is just behind the crew cabin. They will guide it to its latching position on the shuttle using both a camera looking upwards from the airlock, and by eyeballing it from the aft windows. When the time comes to position the just-grabbed Zarya module on top of the Unity module, however, neither of these techniques will work.
Unity has no upwards pointing TV camera.
The first trick on rendezvous day is to catch the Zarya, just like the first step in making rabbit stew is to catch the rabbit. The shuttle will fly a standard chase of the Russian module, from below and behind. But once within a few hundred yards, the shuttle must use an unusual final approach. Because Russian ground commands must reach Zarya, the shuttle cannot fly up from below, since it likely would block the radio signals. Instead, it will fly around in front of Zarya and get above it, and then begin approaching upside down.