Image taken from W.W. Greener's The Gun and its Development, 1910. Image is now in public domain
Click on image to enlarge.
The kneeling position is a more stable stance than the standing position, and is used for target shooting as well as hunting and military purposes. It is the best position for firing heavier rifles, such as those intended for big game like elephants, rhinos, cape buffalo etc. It is also easy to use this position on any kind of terrain. This position also allows the rifleman to adjust his aim relatively easily, should the target move.
In this position, there are three points of contact with the ground: the left foot, the right knee and the right toes (Assuming a right-handed shooter, of course. For a left handed shooter, substitute "left" for "right" and vice versa). Note how the rifleman positions his left elbow-joint just ahead of his left knee, so that the muscle of the left upper arm is in contact with the knee. The left elbow joint should NOT rest on the left knee because the bone to bone contact causes more wobbles. The left forearm is held close to vertical. The right leg is placed at right angles to where the rifle is pointing. The rifle is held across the chest, which helps to steady it more.
It takes a little while to get used to shooting from this position. Beginning shooters might find this position somewhat more uncomfortable, particularly because of the way the right foot is folded. This position can be quickly got in and out of and it also allows the shooter to use concealment behind low walls, rocks etc.