Like the name indicates, the back position consists of the shooter lying with his back on the ground, as the following two images show:
Click on images to enlarge. Images are in the public domain.
When we studied the prone position earlier, one of the disadvantages of that position that we'd noted was that it isn't suitable in places where the ground in front of the shooter slopes steeply up or down. The back position can be used in such conditions. This makes the position useful for long range shooting, where the rifle needs to be elevated to a high angle; and also where the rifle needs to be shot down a steep slope. It is also useful for long-bearded men who do not wish to get mud and dust in their carefully groomed facial hair.
To assume this position, the shooter first sits on the ground facing the target and holding the rifle across the body. The shooter then lies down on the ground on his back and turns slightly to the right, to allow his right thigh to rest squarely on the ground. The shooter then bends his left knee (as in the first picture) or places his left leg on top of the right leg (as in the second picture). The barrel of the rifle is then placed on the side of the right knee (first picture) or left knee (second picture). The right elbow rests on the ground and the right hand is placed on the trigger. The butt of the rifle is in the hollow of the right shoulder and the left hand serves to hold the butt in place. The head is raised off the ground, to view the sights properly. Notice that in the first image, the shooter grabs a bit of his left sleeve with his teeth, in order to keep his head steady.
Like the prone position that we studied earlier, this position requires open land in front of the shooter, as it cannot be used where there are bushes or tall grass obscuring the target.
There are some other variations of the back position, such as the Fulton position, invented by a Henry Fulton, who was a civil engineer and later served in the Civil War as a lieutenant of the 12th Regiment New York National Guard, rising up in the ranks to become a Major.
The Fulton Position.
Click on image to enlarge. Public domain image.
In the Fulton position, the shooter lies on his back and draws his knees in a V-shape and crosses his legs. This allows him to position his rifle barrel between the crossed legs. The left arm is then positioned behind the shooter's neck, thus allowing him to support his head; and the left hand grabs the butt of the rifle. While this looks a bit unusual, bear in mind that this gentleman won the first Wimbledon Cup in 1875 and was part of the US team that won the first international rifle shooting match ever (the original Palma match) in 1876. The National Rifle Association also awards the Henry Fulton trophy, named after the man, annually since 1987 to the highest scorer of the team match in the World Long Range Shooting Championship.
While the back position is not seen much these days, it was more popular in the 19th century. In fact, in the first Palma match of 1876, five out of the twelve shooters in the competition shot lying on their backs and seven shot from the prone position. Those lying on their backs averaged 157 points against 154 points for those using the prone position.