The yaw of the orbiter is a rotation around a defined axis which is through the center of mass of the orbiter from the top of the orbiter through the bottom as displayed in the illustration below.

The Quicktime object movie below illustrates the motion around the yaw axis. The orbiter is shown from two perspectives. The perspective on the left shows the shuttle orbiter with the yaw axis vertical through the center of mass. The perspective on the right shows the orbiter with the yaw axis coming out perpenducular from the screen. Observe the yaw rotational motion of the orbiter by clicking and draging across the picture below.