There are many different shoulder motions, forwards, backwards, out to the side and towards the body. (all-about-arm-exercises.com)
These three regions create a mechanical linkage that is dependent upon one another for proper shoulder motion that is controlled by the upper trapezius, lower trapezius and serratus anterior. (postureanalysis.com)
Tips & Safety: Be sure to keep your arms locked out as you start the explosive shouldering motion at the beginning of each rep.. When you unshoulder the weight, don't comprise back by allowing the weight to pull you down, rather, follow it down and seek to allow the sandbag to softly land on the ground (this will also increase the longevity of your sandbag). (onnit.com)