The rule is a consequence of the fact that the axis of movement for the convex joint partner is within the bone itself, whereas for the concave partner the axis of movement is outside that bone. For this reason, rolling and gliding always occur together.
So If we are moving the convex joint, the glide component always occurs in the opposite direction of the roll. If we are rolling the convex bone part to the right, it would eventually roll off its fixed concave bone partner. Through a glide in the opposite direction, which occurs as the axis of rotation is within the convex bone part, the joint surfaces stay in touch with each other and a greater range of motion is possible.
On the other hand, if we are moving the concave joint partner, the glide component will always occur in the same direction around the axis of rotation that lies in the convex bone partner. If we were only to roll, the movement would be limited very early. By gliding in the same direction, a greater range of motion is made possible.
In a roll movement of the convex partner, the capsule and ligaments on the opposite side to the direction of movement, are stretched.
In a roll movement of the concave partner, the capsule and the ligaments on the same opposite to the direction of movement are stretched.
In a glide movement of the convex partner, the ligaments and capsule are stretched on the same side as the direction of the movement as the convex part is pressing into the capsule.
In a glide movement of the concave partner, the ligaments and capsule are stretched on the opposite side as the direction of movement as the covex part is pressed into the opposite side of the capsule.
If we combine those two movements in a therapeutic intervention, we can maximally stress a capsule by rolling and gliding in opposite directions with the convex joint partner or by rolling and gliding in the same directions with the concave joint partner.
At last, the whole capsule is equally stressed during traction as all parts are stretched to the same degree.
The same is true for a spin movement. In this case all parts of the capsule are provoked like a towel that is twisted.