In 1738, the Swiss mathematician Bernoulli observed that when air moves faster the pressure decreases. The more fast the air moves less it will be its pressure.
  Thus, we can tell that the air is faster in the air flow center produced by the dryer. Therefore, higher speed implies that the pressure in this point will be always lower compared with the others. This is verified because the center is the most away place from the borders of the air flow (where it is the stopped air). The ping-pong ball is always maintained in the air flow center because it is pushed to the center by the pressure difference between fastest air (center) and slowest air (borders). The ball is maintained in suspension because the flow of the ascending air is enough to compensate the ping-pong ball weight. Try it and learn how to experiment Bernoulli!