How does glue work?

I have Animate Pro PLE. I have a basic upper arm shape. A lower arm shape is connected to the upper arm as a child for IK. Both upper and lower arms have their facing sides open, no lines between the joints that is. And I have the glue effect in the network view positioned with the upper arm connected to the glue module’s middle port. And the lower arm connected to the right port. I cannot figure out what the "matte"is suppose to do or where it’s content is derived from. I know what matte means but matting from what? The user guide just says the matte goes into the left port of the glue module.
Any ideas folks?

Thanks.

I got glue to work with two unconnected squares. Glue does not seem to work when the two pieces are linked though. Is this a limitation of the glue effect?