Red dots, which are made by Aimpoint, BSA, Bushnell, and Truglo, do their voodoo by putting the dot and the target in the same optical plane, just as a scope does. But with a scope, you must also get your head on the stock in precisely the right place and then put the crosshairs where you want them. If you don't, the resulting parallax--the apparent movement of a reticle over a target that occurs when you move your head--means you won't send the bullet where the crosshairs are. Using a red dot, you hold your head in any position you please, just as long as you can see the dot, put the dot on what you want to hit, and shoot. More on parallax later.