This Hardware shader is equivalent to the "Use Background" software shader in Maya. It projects a texture from a camera connected to the shader in a flat manner so that 3d moving objects can be used as "image planes".
Let's say for instance that a character need to be animated hiding behind an open door of a car like it can be often see in police movies. The car as been shoot on set and the character is digital. The character needs to be able to interact with the car's door. If a scene has been created with a very rough 3d version of the car and a match-move of the camera, the Hold Out Shader can be used to re-project the camera plate from the match-moved camera back to the rough model. That will help the 3d stand-in car to look almost identical to the live action car (the old texture projection trick basically), it will make the playblast looks much more closer to the final composting and give the animator the ability to better judge the performance. The 3D object doesn't have to be stationary. The car we just used as example could have driven in the frame and then stop. As long as the match move is done properly and the 3d stand-in roughly match the motion of the object in the plate this shader will give you the impression to have your 3d character to interact with the plate.
The zip file includes source code and a README file with instructions for installation and usage.
Let me know if you find any problem.