$multipass
... 
$multipass is a material shader parameter available in all 
Source games. It forces a material to be rendered in separate rendering "passes." This can fix issues with translucent materials, although rendering will be more expensive.
This command is required to allow DX6 + DX7 users to see cubemaps. It has also been used on breakable glass materials.
Todo: What does it exactly do, and for what other reasons is it needed?
