HP 2566C - Manual (Page 63)

Center the stack directly under the falling paper and then push it
against the backstop (Figure 4-6). The backstop guides the paper into
position and helps it fold correctly.
7. Align the paper stack.
Figure 4-6. Aligning the Paper Stack
8. Cluso the stacker door.
Continue to peige 4-7, “Unloading the Paper Stack,” for information
on how to remove paper from the stacker.
4-6 USING THE POWERED PAPER STACKER