Observation is a great tool and the ability to be able to replicate movement from observation is awesome it makes learning new exercises a lot easier because sometimes it is just hard to find words to describe the all the movements that take place in one exercise. But the two downsides to learning exercises in this manner is one you have no way of insuring that the person you are observing has any knowledge regarding basic exercise fundamentals or form which means if the person is performing an exercise with terrible form that form is going to be passed along to you (this is why whenever I workout at my gym I try to stay as strict as I can with my own form because you never know who you might be influencing). Second, because you are not learning the exercise from a knowledgable coach or trainer you do not receive any of the intricate cues or subtleties regarding proper execution of the motion. I use the overhead press and bench press as examples because they are probably the two exercises most commonly performed in a routine, but yet I would guess more than half of the people performing them have never been formally coached how to properly execute the motion.
My reason for addressing this issue is I found two very interesting videos that allow one to see how much difference the simplest adjustment and cues from a knowledgable coach can change the entire exercise in regards to the risk versus reward from the exercise. These videos demonstrate an overhead press specifically, but the technique we describe goes hand in hand with most any pressing motion.
When I coach my clients to perform a press the first cue I give them is to pull their shoulders down away from their ears this depresses the scapula and I then cue them to retract the scapula or squeeze their shoulder blades together this is what we as trainers refer to as packing the shoulder it insures that the shoulder remains stable and cohesive throughout the exercise. But I had no idea what an impact these simple cues have on the biomechanics of the actual shoulder and how much easier these cues make the movement on your shoulder joint check out the videos below to see the difference in the movement of your shoulders when you pack the shoulder and when you don't.