Shrugs are a popular exercise to isolate your traps. It will mostly work the upper part of the trapezius muscle, which is the part behind your neck that "pops out". For a complete trap development, you will need to incorporate another exercise that works the lower part of the traps too.
The traps run all the way from the upper part of your neck, to the middle back. Your traps have two functions, pulling the shoulders up, like the shrug. And also pulling the shoulder closer together or farther apart.
Strengthening the traps will improve shoulder mobility, posture, and might help prevent back pain.
The machine shrug combines the symmetrical movement of the barbell shrug, but keeps the arms in a neutral position to your sides as with the dumbbell shrugs.
The Machine also lets you pack up a lot of weight, which is more difficult when using dumbbells.
Avoid using too much weight, since it can compress your spine.
Keep your shoulder blades pulled down and back, your chest up and your head in line with your spine throughout the entire exercise.
Hold your neck in a natural position. Having your head either too far forward or backwards may cause the neck muscles to perform under the majority of the muscular tension and stimulus; not the traps.
Keep your body and core tight and abs engaged throughout the exercise to protect your spine.
Do not move your hips while shrugging. The movement should be done with your shoulders.