Yeah it is pretty straightforward, and I have seen various shows that show it, so it can't be too secret. Also, anyone who has ever played paintball understands the concept. It is one of those things that is easy in concept, but VERY hard in application.
If you are inside a structure during the day, it is pretty hard to see you in there, especially through a small hole. The "dynamic range" of the human eye is incapable of seeing both at the same time.
So then, shooting through a hole, the closer you are to it, the less cover you have (that is, the precise angle at which a bullet fired AT you and hit you is much smaller). The farther from the hole, the more cover you will have, but the field of view through that hole will be less. You can move around to see different things, but you can't see much all at once.
To get even more crazy/advanced...At first you might think that the hole needs to be at least 2" high in order to see through it with the scope AND shoot through it, but that is not necessarily the case. As we know, the bullet must be on the rise as it leaves the muzzle, which results in a close zero, and a far zero. So for instance, you might be zeroed at 25 yards and then again at 250 yards, depending on cartridge and scope-to-bore axis distance. If things time out right, you could theoretically view a target and shoot through a hole hardly large than the bullet diameter, and hit your target. Of course, the shorter distance from sight axis to bore axis helps.