If you've seen a hen squat into breeding position you may have noticed that she'll tip her rear end upward. This "presents" her vent as a target. The rooster mounts her and dips his rear end downward toward the waiting vent of the hen. He drops the sperm and she accepts it. This all happens very, very quickly.
After a dismount and a shaking of the feathers, they go on their way.
The hen, however, chooses whether she wishes to excrete the sperm, thus rejecting the mating. It doesn't happen often, but it does happen. The hen will position as if she's about to poop, but she'll express the sperm onto the ground. If she shakes and walks away, she's keeping it and it will be available to her in fertilizing the next 12-15 eggs.