Pressure Compensated Flow Control - Part 1 

Yes, a little like a governor. This valve is an inexpensive way to achieve fairly accurate cylinder stroke speeds or hydraulic motor shaft speeds, even when pressure values are inconsistent. A simple needle valve flow control cannot accomplish this level of speed accuracy during pressure fluctuations.

I have had good results with a pressure difference of about 100psi. Each pump should be equiped with a check valve on its output. Remember that in this configuration the lowest adjusted pumps will supply most of the flow and the higher ones will vary in volume the most; so wear can be unequal. But, if adequate filtration and cooling are provided on the system, there should be no problems.

The change in the cross section area helps to reduce or increase the pressure drop due to the load pressure but there's no change in the flow but how? The pump pressure is constant 2000 PSI which means a constant flow across the relief valve. (The pump and relief valve is not included in the video but i think you can figure out how the system work now)

You are right if it is a constant flow source , then you cannot control flow . Continuity equation holds true for steady state conditions ..when you change the area of needle valve to new position the system reaches new steady state value with different inlet pressure . This pressure is used to reduce the displacement of the pump and hence the flow reduces , the system reaches another steady state condition. I have pondered long over this ,i find the concept of having a needle valve to control flow as difficult to digest. Going by physics, needle valve cannot control flow if it is a constant flow source.But always it is taught in hydraulic classes that way with no regard to what energy source we are working with . Any better answers would be appreciated.