HP LineJet Série Guide De Référence Rapide page 72

Chapter 1
Power Stacker Option
4.
Set the desired paper length (5-12 inch range), as follows:
Grasping the paddle shaft, push or pull toward the front or the
rear of the printer, setting the desired paper length by aligning
the indicator notch on the bearing bracket with the paper length
indicator.
5.
Continue with the "Loading and Starting Paper Stack" section.
72