The science behind this assessment is twofold (this the first of two responses). To fully comprehend this ROM assessment we first must understand what exactly is happening at the tissue level. All tissue in our body follows one simple rule when a stress is applied (a load) the tissue (muscle, tendon, ligament and even bone) will undergo strain or lengthening. So using this example the stress is not the exercise but the weight of your upper body as you lean forward. This stress or load being placed on the muscles of your posterior chain (back, butt, and hamstrings) is gravity, in essence pulling you downward.
What we then measure is the strain or length of that the load produces. So before your exercise you get on the box and measure your body’s ability to lengthen or undergo strain, you then do your exercise and come back and perform the ROM assessment. With everything being equal (assuming you didn’t lose or gain weight during the exercise even a little loss of sweat will not affect it) your strain/length then shortens. So same load as before but now the strain decreases? This relationship between stress and strain is what we define as “stiffness." The more stress and the less strain the stiffer you are, or the less stress and more strain the less stiff you are. In this case the same stress and different strains leads to an increase in stiffness. (See picture for all the visual people out there) So the question is what physiological principle leads to this change in the tissue that makes it stiffer??