I knew about this vision trick from high school science class and have used it on occasion. Except I find it easier to pinch my thumb and forefinger of both hands and then put the four fingers together to form a tiny hole in the middle. I recall the tiny hole forces your eye muscles to tighten more and improve your eye focus.
Try it with some small text on paper about 5 feet away and you should see a difference even if you have 20/20 vision.
Quote via Hacker Zen:
Make your hand into a fist and look through it, as if you were looking through an invisible telescope.... The tiny hole only allows a narrow beam of light to enter the eye... [reducing] the size of the circle of confusion on the retina and increases the depth of field.
Via LifeHacker