A gymnast using a set of paralleled bars.
A gymnast is performing a routine on the bars.
A gymnast is performing using a set of paralleled bars.
A gymnast using a set of bars is performing a routine.
Sentence one states that the gymnast is using a set of parallel bars. "Using" indicates that the man is interacting with the bars in a way that is useful. Because the only way in which to "use" parallel bars is to perform gymnastic routines, this would indicate that the gymnast is performing a routine.