Stand the plywood underlayment panel vertically with its 4' edge on the floor and the panel face towards you (the staple/nail pattern will be facing you).
Grip the top panel edge with your right hand and firmly place your left palm against the face of the panel.
Pull down with your right hand approximately 3" while simultaneously pressing forward with your left palm approximately 3". Hold this position during inspection.
With the plywood underlayment panel in this flex position, thoroughly inspect the entire panel surface for irregularities, which will look like "bubbles" or "creases." In the rare instance you find such an irregularity, do not use the panel. Set it aside, and contact Patriot Timber Products immediately for a resolution.