Conceptually, it's a simple concept. Assuming you have perfect form, there should be a straight line going through your shoulders through your bow arm to the bow. There should be another straight line going down your string forearm through the arrow to the bow. These two lines meet in a V at the bow. Since they are coming from different directions, there is no particular reason why the string should hit your bow forearm. Of course, sometimes it does.
Pragmatically, most people are able to shoot the bow without the string ever hitting their bow forearms, so we know it can be done. Mostly, if the string hits the bow forearm, it's because the shooter has moved the bowarm out of the straight line into the path of the string. You can do this by rotating your bow hand too much toward the center of the handle, or if your elbow joint is facing up rather than sideways. You can also do this if your bowarm elbow hyper-extends (can be straightened out beyond 180*), which moves it into the path of the string.
However, take comfort in knowing that a number of expert archers always use an arm guard.