BMX racing generally involves short bursts of speed and power at very high rpms. Riding at a very high cadence requires particularly good coordination. (Coordination and technique is one area Lance Armstrong started working on in 1993 that has allowed him to ride at higher cadences than his competition and to dominate cycling for nearly a decade) While one can try to "muscle" ones way to this goal, it is very difficult to do so and that power could be better spent pushing the wheel instead of fighting your own legs. When learning, PowerCranks generally causes the rider to slow their cadence down (riders find out that high cadences are less efficient and most PowerCranks users are long distance athletes) but PowerCranks can and have been used to work on improving high cadence coordination to increase sustainable cadence and high cadence power.
Examples of what is possible include the report of a track racer who, over 6 years, was able to increase his top rpm from 180 to 240 and his top "sustainable" rpm increased to over 200.
If you need to increase your high rpm efforts integrating PowerCranks into your training can greatly improve your abilities.