The other angle of viewing the roll is from the side.  When reduced to geometric terms, the rolling pigeon is actually a revolving triangle.  One side of this triangle is formed by the back of the neck and head as they are thrust backward to meet the second side of the triangle-the tail. Each of these two sides hinges on the critical "base" of the figure, which is the bird's back. (Fig. 3) The pigeon’s back is actually a number of vertebrae fused together to form one solid bone.  The back of the pigeon does not flex during the roll or at any other time.

The neck, and the tail actually hinge upward toward each other from their respective points of attachment at each end of this backbone.  During the revolutions of the roll, the head is continually pressing backward trying to meet the upturned tail. Counteracting this tendency to touch is the equally strong centrifugal force radiating outward in all directions from the center of the gyrations.  Therefore, this rolling triangle is slightly open at one point-but the basic figure is maintained until the bird can no longer resist the forces acting on it. 

At that point, the pigeon snaps out of the roll and returns to the kit or else it continues gyrating, though at a lesser speed, and plunges to its death. At rest, and even during straight ahead flight, the pigeon's center of gravity-its physical "balancing point" is somewhere below and behind the point of attachment of the wings to the body.  When the bird starts rolling, however, this center of gravity shifts up-ward to the top of the back, or even slightly higher, to that imaginary point around-which the pigeon rotates.

Going back to the image of the triangle, this center of revolution -is approximately at the center of the triangle.  As the triangle rotates around this imaginary midpoint, the visual effect of a hole is created.  The size of this hole is determined by the size of the triangle, which is most closely related to the length of the back.

Although this hole is not often seen it is there, nevertheless, when any above average bird performs. Whether this "hole in the side" is actually seen by the viewer is complicated by the presence of the ever-beating wings. The wings are attached to the leading point of the base of this triangle where the neck hinges to the back.  As they beat up and down, they are continually flashing back and forth across this hole in the triangle.

Whether the wings distort the image of the hole that is created is dependent upon how those wings are feathered.  The critical factor here is the length and width of the secondary flights and also the quality of that bit of feathers between the secondaries and the body. These feathers are sometimes called the tertiary flights.  Excessive secondary and tertiary feathering will obscure the hole.

STYLE AND VELOCITY

One of the current points of controversy in the Roller fancy is whether the hole in the side is actually the mark of ultimate velocity.  This controversy has formed around the general statement that the highest velocity Rollers do not or even should not, show the hole in the side of the roll.  Before addressing this question, it is important to distinguish between rolling "style" and velocity.  My definition of style includes cleanness of outline of the small hole in the side and also clarity of the external circumference of the falling, spinning ball.  

These are the two basic ingredients of style and, on the face of it they can be distinguished from velocity of the revolutions. Among Rollers, one can find a number of combinations of style and velocity. A slower Roller of a certain minimum velocity could show a hole if its lack of secondaries permitted the feature to be seen.  On the other hand, a pigeon with more "cover" would not show a clear hole, or any hole at all, but still be capable of higher velocity revolutions than its more "stylish" loft mate due to greater chest muscling and more aerodynamically efficient flight feathers. 

However, the final answer to this question is only possible when one compares pigeons capable of equal rolling velocity.  The only difference would be that one has longer and wider secondaries and tertiaries.  Longer, wider secondaries increase the overall wing area. During the roll, the amount of wing -area contributes directly to the degree of air resistance and air turbulence, which slows the roll.  The pigeon with longer, "drag" inducing secondaries which obscure the hole in the side would thus have a slower wing beat and a slower, however imperceptible to the human eye, speed of revolution.