A flying broomstick is not simply a “normal” broomstick pressed into service as a mode of transportation. The flying broomstick is a magical item with built-in charms. The earliest known evidence of a broomstick enchanted to fly dates to 962 AD; brooms are thought to have been chosen because they are easily transported and concealed from Muggles. As time passed, they would also prove to be conducive to playing a number of sports.

Flying on a broomstick, particularly one which accelerates very quickly must necessarily involve some form of magic protective field holding rider to broom and shielding against wind and inertial forces. Without this, it seems unlikely that anyone could hang on at this high speed. It also seems likely that flying a broomstick doesn’t involve actually sitting on the handle, which would be uncomfortable indeed. As a matter of fact, most illustrations we see in the books show the rider floating a little ways above the handle. The spell which creates this effect is called the Cushioning Charm, invented by Elliot Smethwyk in 1820.