In a conventional spindle motor fan, a "dead" area is created right behind the bulky motor, this prevents air from passing efficiently directly below the fan (which is where the core of the CPU is located at if you think about it) not giving the hottest part of the heatsink the same airflow as the cooler parts get. There still is a small dead area, but it's cross section is so small that it is a significant improvement over conventional fans. What this translates into is lower RPMs (and lower noise) while providing equal if not better airflow to the heatsink.