Well... not an expert, but...
I've heard here that some people focus on the retinal burn of the kasina when they close their eyes. For me I have a blue circle on an empty white wall I stare at. I will see a bright spinning orange dot about half the size of the kasina when I close my eyes. It will disappear and reappear less intensely several times then go away.
What I've found is the slightest eye flickers/movements tend to be what force the after image away, but I'm not 100% sure about this. I've tried relaxing my eyes more but it seems impossible to fully control these.
This seems to me like the first vipassana jhana, and a fairly good description of mind&body, cause and effect and 3C.