What do the neck extensors do?

They are muscles the pull your head back. So if you are walking along, minding your own business and hear a fighter jet cruising through the sky, when you tilt your head back so you can look up to see they are about to shoot some terrorist down, you will be using your neck extensor muscles. Conversely, when you realize that its actually just a 747 taking off, but you trip over a curb because you weren't looking were you were going, you will probably look down in preparation for impending fall. Looking down uses your neck flexors. As a rule of thumb, flexor and extensor muscles always work in opposition and generally occur in a plane that is perpendicular to the body in its anatomical position. Flexors reduce the angle of the joint (ie biceps brings forearm closer to arm) and extensors increase the angle of the joint (triceps brings forearm away from arm).