What is motive flow and why is it used in some aircraft hydraulic systems?
Motive flow is required in design of ejector pumps in fuel system. Ejector pumps (also known as jet pumps or eductor pumps) have no moving parts and therefore represent a highly reliable, low cost means to move fuel in an aircraft. All jet pumps contain the physical characteristics illustrated in below figure.



In this pump, the motive flow comes from a high pressure source such as a primary boost pump, or the returned fuel from the engine. When the high pressure motive flow passes through the throat it creates a suction which sucks fuel from its neighborhood and the total fuel flow are sent to another location. The overall efficiency of an ejector pump is low when compared to electric motor powered pumps but they are frequently used due to their extremely long life, high reliability and low cost. 

The ejector pumps generally are used for two functions:


Ejector pumps are often used to provide the primary fuel boost function in smaller transport aircraft applications such as business jets and regional aircraft. 
In order to minimize the unusable fuel, scavenge pumps are often employed to suck up fuel from the remote corners of fuel tanks and to discharge this fuel at the inlet to the main feed pump(s). Ejector pumps are used for this purpose and the motive flow for these scavenge devices may be taken from the feed pump outlet if other motive sources are not available.


Note: Most of above material are from: Langton, R. (2009). Aircraft fuel systems. Chichester, U.K Wiley.