Thank you for your detailed response. Let me clarify. When you use either jit.render or jit.world, a single light source is included without explicitly specifying. With jit.render, that source is moved with the light_position attribute:
Martin, in your example, when I disable both light sources, there is an implicitly declared light source still operating; but if I enable one of the explicit sources, the implicitly declared source disappears. So this might be my solution, but it would still be good to understand how to manipulate the implicit source.
I have no experience with the light_position attribute of jit.gl.render.
I would recommend using jit.world with explicitly declared light sources. (Adding one of them to a world will disable the default light.)
The light type "directional" may not show changes to all positions as far as I understand the physics behind it.
For an example of lights you might also see the Jitter sneak peak video at around 2:12