This is a video I made last year and never put it up on the blog. This is one of my favorite sequences to teach. I think the muscle work required to pull off this tracking exercise exactly mirrors the work of the arms in downward dog. The Internal rotation of the forearm and the external rotation of the upper arm is simple yet complex.
There should be a lot of work towards the midline of the body. As you bend and lower the arms the elbows need to move in to come down in line with the wrist and shoulder. Coming back up requires the same pull towards the midline for the elbows to lift straight up.
There are numerous limiting factors for students. Tight shoulders and weak lats will make this difficult as well as a lack of core tone. You need power in the belly to help solidify the shoulder blades onto the back in order of the arms to track freely.
Give it a try and have fun!