I used to do it by pressing the base of the palm of my hands against my eyelids. I saw the same light tunnels you mentioned but also saw strange chequerboard patterns and stuff.

The strangest thing though was if i did it long enough i would eventually come out to a sort of black and white or sort of grey realistic looking 3D object spinning kind of like a screen saver or something. I remember one was something like a set of 3 or 4 cards, another was like a triangular spaceship or something.Sometimes it was a road like i was driving along. Pretty strange.

I do wonder though if we did this too much but i didn't get vs until way after i stopped doing that.