FBarra said:
Underlighting is an old technique where the cel is mixed with lighting.
Not just in hand drawn animation, but also in live action productions, like the Star Wars lightsabers.
This is called T光 (透過光) in Japanese.
It has less to do with "sakuga" and is more of a compositing thing, so I'm unsure if it's something that would apply to this booru, but I think it's an interesting thought.