^^^The bracket (also called a console) rotates along the axis of the through bolt until it's tightened down against the center of the bushing. Aside from cushioning that pivot point, the bushing also dampens the swingarm motion of the trailing arm by way of the rubber twisting as the arm goes up and down. When a straight edge is used as prescribed, it ensures that the rubber isn't already twisted and tensioned while the suspension is inactive; it sets the console's tightened position against the bushing to the correct "preload". In other words, the straight edge mimics how the parts line up at static ride height.
Same principle as the front lower control arm bushings: you have to get the car back down on the ground before the lubricant dries up, so the bushings tighten down on the control arms in the proper position (static ride height). Otherwise, your arms won't behave as designed, and your bushings won't last as long as they should. In the case of the rear, there is no lubricant involved; instead you tighten the through bolt to lock the console into position. But since the whole assembly mounts inside a cavity in the car's chassis, there is no way to reach this bolt when the console is already installed, and the car is back down on the ground. It's an important step that people often overlook when replacing their RTAB's.
While the real alignment tool attaches to the wheel hub's center (for convenience), the makeshift straight edge I used does not. The important thing is that the imaginary line you're creating ends up 8mm above the center of the hub, and lines up with the underside of the console's mounting flange. To illustrate the point, I superimposed an improperly aligned console (in green) over the original photo: