Lets say you have a backhoe that the hoe wont lift as much as it should. If you put a flow meter inline on the main pressure hose coming out of the pump you can check pressure and flow. The system will have a min spec of what GPM at a set PSI the pump should maintain. Lets say it should be 50GPM@3500PSI you check it at no load (usuaully less than 100PSI) and it's pumping 60GPM. Looks good at first, but the GPM starts drop in as you increase the load (flow meters have built in adj restrictor to load systems) the gpm drops. It's possible that the flow completely stops 0GPM and the pump be making say 2500PSI the machine will stall. Thus can you have pressure with no flow and the system still work till you till the load gets to heavy. Make sense 
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