The USPS is tightening up the Deflection standard for Automation Flats effective June 7, 2010.  Due to the way automated Flat sorting machines process mail, a Flat must have a degree of rigidity in order to be processed without damage.  Deflection is the measurement of this rigidity.  This is separate from the requirement that Flats have a minimum degree of flexibility. 

For flats over 10 inches long, the flat is placed on a surface with 5 inches of its length extending over the edge.  A ruler is places on the flat parallel to the edge of the surface and a 5 pound weight is placed on it.  The flat cannot droop more than 3 inches.  This test is then performed again, turning the flat around first.