$ambientboost
... 
$ambientboost is the QC command. Indicates the model be rendered with the ambient boost heuristic.
In order to keep a model from becoming too dark in some situations, the heuristic boosts the ambient cube contribution in cases where the ambient term is otherwise overwhelmed by any direct lights. To use this, include $ambientboost in your .qc files for a given model and the boost heuristic will be applied to the ambient cube contribution of shading.
This technique was first used on Alyx in 
Half-Life 2: Episode One.
Syntax
$ambientboost
