Strength
Strength and explosive power are required to perform some of the key moves on the still rings. When performing a move like the iron cross, a gymnast is starting without momentum and can rely only on his strength. Once the gymnast gets into the position in which he has fully executed the move, he must hold it for two seconds before releasing and going on to the next move.
Balance
A gymnast must have the kind of balance that allows him to hold his weight with one arm and then extend his legs so that he is parallel to the ground. As with any still rings move, this takes superior strength. But when performing any move like this, balance will allow the gymnast to keep the rings still and execute it completely.
Quickness
When performing a move like the back uprise to a handstand, you also need to show that you have the quickness to transition from one move to the next. This is especially critical when you are driving your heels to the gym's ceiling. If you are not quick with this move, your heels might not get to the point that they are perpendicular to the floor. This will leave you in a bent over position that will cause point deductions from the judges.
Timing
What separates an average still rings performer from a good one and a good performer from a great one is the timing of each move on the still rings. Strength might be the telling factor, but if you don't time your moves and transitions perfectly, difficult moves get much tougher and might be impossible. There are some swinging movement that requires significant pressure from the hands on the rings to get the rings moving and then stopped in an instant. This requires timing and understanding of the apparatus.
