BMX racing generally involves short bursts of high power and speed at very high cadences.
Maximum power is determined by your training and technique
High cadences are facilitated by efficient technique
Training with PowerCranks trains more muscles to use better technique.
Riding at a very high cadence requires particularly good coordination. Take your chain off and see how high a cadence you can get to at zero power. You are limited here by your coordination, not by your muscles ability to generate power. Another thing that might limit you is what crank length you are using. While one can try to "muscle" ones way to a high cadence, it is very difficult to do so and that muscle power could be put to better use pushing the wheel instead of fighting your own legs as your technique deteriorates at high cadences and as you get tired. PowerCranks teach the coordination necessary to reach high cadences and to give all the used muscles the endurance necessary to maintain high cadences. Well trained PowerCrankers using appropriate length cranks should be able to achieve a cadence of 230-240 or more. If you cannot achieve such cadences now PowerCranks probably have something significant to offer you.
In short, sprint activity, sports, like BMX, the victory requires very high power but frequently goes to the rider who slows the least when tiredness sets in. PowerCranks can and have been used to work on improving high cadence coordination while, also, increasing sustainable cadence and high cadence power in BMX and other sprint distance athletes.
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 and sprint power integrating PowerCranks into your training can greatly improve your abilities.