Before the advent of powerlifting in the 1960s, overhead pressing fulfilled the role of defining strength. Putting large amounts of weight overhead comprised a majority of athletic training, and pressing body weight became a rite of passage for aspiring strength athletes. Of the many ways to press a weight, the two most common -- the military press and the behind-the-neck press -- develop the most shoulder strength.
The overhead, or military, press heavily recruits the shoulders and triceps. There is also some recruitment of the upper back and chest for stability when executing this lift. The muscles of the rotator cuff stabilize your arm, and in a more balanced manner than the bench press. The military press recruits the external rotators more than pressing in horizontal plane, which primarily recruits only the internal rotators. The overhead press may be done seated or standing. When you perform the overhead press standing, you recruit additional core musculature for stability.
Pressing behind the neck is traditionally used to ensure pressing in a completely vertical path. This exercise requires a bit more flexibility in the shoulders than pressing from in front of the neck. You do not have quite the same leverage as when pressing from in front, so when pressing from behind the neck, you need to reduce the amount of weight you use. This movement is viewed as more dangerous according to gym lore, but no evidence exists to support this. Exhaustive research reveals that the most common injuries occurred when pressing behind the neck for the same reasons they occur in other exercises -- poor exercise technique.
Differences Between the Two
There is not much difference between the press and the behind-the-neck press. It is easier to press in a straight line without leaning back when performing the behind-the-neck press. This press does not require as much flexibility, and you can use more weight. It is easier for novice trainees to learn to press from in front of the neck, and behind-the-neck pressing should follow only after you achieve shoulder flexibility. Both exercises are tools to achieve shoulder strength, and their usefulness lies exclusively in the hands of the user.
Shoulder Strain Risk
There is a degree of difficulty when performing any lift, and pressing is no exception. The biggest issues are poor technique, followed by inflexibility. If you have a long history of bench pressing without doing anything for your external rotators, you will have trouble with the behind-the-neck press. You can either stick with pressing from the front or strengthen your external rotators. You may also have to limit the range of motion on both exercises, depending on your flexibility. If this is the case, you need to stretch your shoulders extensively, although sometimes simply holding a light weight in the bottom position of the press can help with this. While you can press through a partial range of motion, a full range of motion is optimal.