Blending modes --> Silhoutte Alpha )on a duplicate of the head below the actual head.
To mask you need an alpha channel. If you just have a stroked shape layer for the head then you don't have an alpha anywhere but over the stroke.
You'll have to fill the shape and either duplicate it and use the duplicate as a track matte for the arm layer or you'll have to set a mask on the arm shape layer that's exactly the same size and shape as the head.
If you need the head shape not filled then you can turn off the fill for the head and still use the track matte.
If you can use the head shape with a fill then you can just stack up the layers in the right order and the hand will be blocked by the head shape.
There are so many ways to do this kind of thing that a screenshot would be very helpful.
Here's a setup with a track matte and a hollow circle for the head. I've selected the hand so you can see the paths. Notice how the stroke dissapears behind the head layer.
Thanks guys! I will try this now and see how it goes