This is the response I gave to a similar questiona similar question
....This is designed to slightly pull your shoulders back and evenly distribute the weight across your back (i.e. so that the weight doesn't simply sit on your lower back). Pull these tight so that you feel the weight move up slightly (to the middle of your back) and doesn't sit too low. You should feel your shoulders being slightly pulled straight and backwards (imagine your shoulder blades being pulled flat with your back, this should naturally adjust your posture so your shoulders are flatand your back straight).
