Depending on your grip you can vary which muscle is primarily used in this exercise. A wide grip directs the load on your deltoids while a narrower grip directs the load to your traps. If you are trying to target your traps I suggest using a curl bar (EZ bar) as there is really only one grip you can use plus the shorter length makes it easier to balance the bar.
Start with the bar below your waist and raise the bar upwards to right below your chin. Lower the weight back down in a slower controlled manner. Repeat the process.