How It Works
Pic2 shows how to configure your thumb and the next two fingers to create a small triangular opening that you hold about an inch from your eye. Narrow the opening until your view is in focus.
You are effectively creating a pinhole lens. This gives a slight magnification and significantly increases your depth of feild (the range that is in focus). It does this by greatly limiting the amount of light going through the lens in your eye. It is like adjusting the F stop in a camera.
The 3 finger reading glass works best in brighter light. To get a wider feild of view, you can use both hands held up to both eyes (pic3).