The serratus anterior is that cool-looking muscle on the side of the upper abs area, sometimes referred to as the "boxer's muscle." It isn't technically an abdominal muscle, but it completes the look of a muscular, shredded torso.
Problem is, it's kinda tough to train directly. In fact, many strength coaches say to forget about targeting the serratus and instead just get your body fat very low and do lots of overhead presses and pullovers, which bring the serratus into play. Not bad advice, but you can directly target this muscle group. Here's how:
This one takes some practice. Adjust your position until you can feel the serratus contract. Once you develop that mind-muscle connection, it'll be easier.