It's not a difficult concept.
Light goes in the bottom on one side, is reflected up and back into
your eye, and presto! You have a miniature's eye view of the battlefield.
Until you take a look through
one of these things, you will think you don't need one. Do you have
any idea what the difference is between your titanic point of view
and the puny miniature's?
Well, the difference is
huge. Shots you thought were clear are impossible. Shots you thought
you could not make are clear. Cover and terrain, even the slightest
hill, makes a difference, and you know what? It's quick and easy.
Solve arguments with just
a shrug and a hand-over of the periscope - when gamers actually get
down into the dirt (well, their eyeball, anyway), they get a whole
new perspective on their favorite game.