Describe proper running mechanics. describe the position of the arms, torso, head and shoulders and explanations as to the importance of each.

Running on your toes, don't plant into your heels while running -> bad for knees and shins. 
ALWAYS STRETCH AND WARM UP BEFORE RUNNING -> take care of your muscles 
Try to stay light on your feet, don't step forcefully, running on toes helps with this. -> For your ankles knees and shins 
Knees should come up while running, especially running uphill. High knees but not too high. -> also for the welfare of your legs. 

Arms are swinging at a 90 degree angle at ALL TIMES. 
Arms swing minimally. Extra movement is not needed. 
Shoulders are relaxed and low. Do not bring your shoulders up. 
Arms are nice and low by your waist, do not bring your shoulders up 
Torso is not moving side to side, but facing straight ahead with NO MOVEMENT. 
Nice straight back. Make your chest broad and big. Not only will it help with your back but help with bringing in more oxygen into your lungs 
Head is looking ahead with NO MOVEMENT. 
Hands and fingers are curled and RELAXED 

Pretty much your upperbody is RELAXED, STILL and NOT MOVING. Only your arms are swinging that's it. You don't want any unnecessary upperbody movement. It is a waste of energy and keep your body relaxed, don't stress your upperbody and back.

Sorry again, hoped I helped! Please mark as brainliest and have a nice day!