When parallax is properly set for the distance involved, you can make small movements of your eye left, right, up, down, and assuming that you don't move the gun, the crosshairs will stay in the same place. When the parallax is improper, if your eye is directly aligned with the crosshairs and target, it doesn't matter, but if you shift slightly left, right, up, down, the crosshairs will appear to move. Depending on the parallax setting, the crosshairs may move in the direction of eye travel, or opposite that direction. Non-adjustable scopes are set for a compromise distance, that's one difference between a shotgun or black powder scope and one intended for use on a high powered rifle.