The turbomolecular pump (or turbo pump) is essentially a fan (rotor) which rotates at extremely high speed, e.g., 50,000 rpm.  It uses a large number of fan blades in stages on a highly balanced rotating shaft, and it actually works by pushing molecules in one direction.  However, it cannot achieve these high speeds if the pressure is too large, for then it would need tremendous power in order to move around so much air or gas.  So instead, it is used to achieve high pumping speeds only at low pressure, i.e., in the molecular flow range where the pressure is below 1 torr and molecules primarily hit chamber walls and solid surfaces long before they hit each other.  With the fan blades swiping at the molecules often (high fan speed), they hit molecules in the direction of desired pumping, leading to lower pressures on the pump's inlet side and higher pressures on its outlet side. 

The speed (or conductance or capacity) of a turbopump depends primarily on the size of the rotor (i.e., how big are the fan blades) and on the speed of their rotation and power of the motor driving them.  If the pressure moves into the viscous flow regime (>1 torr), the viscosity of the gas slows down the speed and therefore the capacity of the turbo pump.  This has two consequences.

First, the turbo pump becomes ineffective above about 1 torr, meaning that the effective pumping speed at higher pressures drops gradually to zero.

Second, as the effective operation of the turbo pump proceeds, the pressure at its outlet (behind the pump) increases, causing a small portion of the gas to travel upstream (against the fan blades toward the inlet side of the pump), thus limiting the extent to which the turbo pump can achieve low pressures at its inlet.   This effect can be minimized by careful mechanical and dynamic design.  Also, as the outlet pressure rises, it slows the rotor speed and pump capacity.  Therefore, the outlet side of the turbo pump must be pumped by yet another pump capable of operating at higher pressures, namely the mechanical or rotary vane pump.