Custom shapes for health bars

[Uriyel]

I've been searching these forums for hours but unable to get a clear cut explanation on how this is done. The example that sent me searching was here.

It's mentioned that this can be achieved in pitbul and stuf. I've used pitbul in the past, but how can you use a custom image to get out of that dreaded rectangular bounding box?

Is it a trick of using two images to mask out different sections of a frame?

Unkn had a link to the most abstract bars I've ever seen.

I'd love to know how this is done.