Is center of mass or radius of spin more important for quick rotation?
The following discussion begins after rotation has been initiated.I was once asked if the pike position had to be compact for quick rotation. My answer was no because the radius of spin didn't get any smaller by folding over and touching the head to the knees. As long as all body parts were within that radius it didn't make much difference except for the purpose of judging.
Most people understand that making the tuck position as compact as possible shortens the radius of spin, thus allowing quick rotation. Some teach a split tuck for the purpose of achieving quicker rotation. However there is a point in a split tuck where the center of mass is spread sideways on the spinning axis but doesn't bring it any closer to the axis. Therefore, at that point the speed of rotation remains constant. Divers must be aware that there is a penalty for using the split tuck. They must also understand that the more they spread the knees, the more noticeable it becomes from a profile view.
When achieving a compact pike position, the center of mass must be as close to the spinning axis as possible. Use of the tuck position allows for quicker rotation than the pike. The center of mass (a point where the body weight seems to be concentrated) seems to play an equally important role as the radius of spin (a point half way between the toes and buttox).
A small radius is necessary for quick rotation but bringing the center of mass closer to the spinning axis "theoretically" accomplishes the quickest rotation desired. In order to gain the benefit from this compact center of mass, the diver should hold the pike at a point just behind the knees. Some divers have been observed grabbing the ankles or the mid-calf area. There is a shift of the center of mass away from the spinning axis when a diver moves the hands from behind the knees toward the ankles. The affect of this movement toward the ankles is that the somersault will rotate slower. At my level of coaching it seemed more important to focus on the initiation of rotation since many divers at this level have some difficulty understanding what causes rotation to occur and when to initiate it. In any case a compact position would be most desirable.
It appears that the above principle is also true when twisting. We see all kinds of arm placement during twisting dives. According to the rules that is acceptable because the arm position is the choice of the diver however some of those placements are not as effective as others. One arm behind the head and the other across the chest or upper abdomen may not as effective as both arms being held in against the chest. Both arm positions may have the same radius for twisting but the center of mass is closer to the twisting axis when both arms held in against the chest. Figure skaters demonstrate this principle when they perform pirouetting twists on ice.
As a coach, I do not recommend any one twisting style over another but try to move a diver toward a style they appear to be closest to. In any case they are coached to show the most compact twist position possible.