They put the airflow measuring equipment in there. Pressurized air flows in both sides and exits through a hole in the stem. In the stem, the mass molecular velocity is read by a series of variable pitch aeroscoptic vanes. The air then exits through the steerer tube and out the bottom of the fork.

It has been observed that some less-aerodynamic frames exhibit a phenomenon known as sine wave gaseous discharge. This is caused when the exiting air becomes turbulent in the steerer tube. As such, a noise resembling flatulence may be emitted at the fork crown. Many believe this is why Columbus steerer tubes are rifled; an early attempt at minimizing harmonic airflow pulsations through the steerer tube.

-Kurt