Voluminance calculates the amount of light each vertex receives taking into account the distance traveled inside a mesh. A longer distance traveled inside means more of the light got absorbed resulting in a darker vertex color.
To view the result in the viewport enable the 'VertexColor channel display" in the object's properties dialog. For rendering you can you use the vertexcolor-map to drive material parameters.
Basically this is a very quick way of getting a sub surface scattering look and feel that renders in seconds.
Usage from scratch:
1) create mesh
2) in mesh properties enable 'vertexcolor channel display'
3) apply VolumetricToVertexColor modifier
4) pick a light
5) adjust multiplier to set the effect's strength
(A quickstart demo scene is attached as well.)
Tip: For organic meshes use effect on a low res mesh an then apply a turbosmooth modifier for faster results.