Turbomolecular pumps use a rapidly spinning turbine rotor to push gas from the inlet of the pump towards the exhaust. Most turbomolecular pumps employ multiple stages consisting of rotor/stator pairs mounted in series. Gas captured by the upper stages is pushed into the lower stages and successively compressed to the level of the fore-vacuum pressure. As with the Diffusion pump a mechanical fore-pump is usually employed to reduce the exhaust pressure.
The maximum compression varies linearly with circumferential rotor speed. In order to obtain extremely low pressures on the order of 10-10 torr, rotation rates of 20,000 to 30,000 revolutions per minute are often necessary. Unfortunately, the compression ratio varies exponentially with the square root of the molecular weight of the gas. Thus, heavy molecules are pumped much more efficiently than light molecules. Most gases are heavy enough that this isn't really an issue, however hydrogen and helium are not pumped efficiently.