I just figured this out for myself. I found a few things lacking in the description of this technique.
First, when starting it with your thumb you need to roll your thumb onto and off of the rotor quickly flicking it with the tip of your thumb to gain extra speed.
Second, when spinning the powerball you naturally have a tendency to roll your wrist further out when roll inwards toward your body as apposed to outward and away which tends to be a shorter ran of motion.
So when attempting a thumb start roll the powerball equally with your wrist so that the range of motion is more of a large circle. You should feel the rotor start to catch and then you can speed up as you wish, just keep in mind when the rotor spins slower you need to provide more range in motion for it to find it's own rhyme.
Hope this helps