This is actually very interesting, I am glad this has been posted. As a kid, I would cover my eyes with my hands to create darkness, or sometimes put my face into a pillow or even do what Tesla did in putting a slight pressure onto my eyes with my fingers.

I would start to see shapes and just be thinking I could change the color and contort these shapes. Strangely, I never knew how I did it. If I tried too hard it would not work, I would have to let my mind run on its own, and give it a slight nudge every now and then consciously.

In college, when I was trying really hard to visualize a project I was working on I would do the same, I would put pressure on my eyes and try to make it as dark as possible so that I could see what I was trying to imagine, and it helped!

I had no idea others do this as well, this is very cool. Some forms of meditation will be very useful for this I think.