The simple hydrodynamic definition of pressure is resistance to flow. All a Boris meter does is give you a back pressure reading based on the restriction of the needle. The more you close the needle the higher the number goes. So if you have a blocked fuel filter it will record a leaner (higher) number because it won't flow as much. You are correct that my newer twin pump Boris meter shows this more clearly than my old single pump one did because it has a higher flow rate thus a proportionately higher number on the gauge. On an average race day I blow everything out every other heat anyway, especially when the ambient temperature is high.
An example would be a garden hose. With no spray nozzle the water will shoot out a large stream of water maybe 3'-4'. Add a nozzle (ie., fuel filter blockage)and that restricts the opening and the water builds pressure and shoots a lesser volume of water further due to back pressure.
That is why pumps are rated in gallons per minute, cubic feet per minute or inches of vacuum (in the US) instead of increments of pressure. An air compressor builds pressure as the air builds in the tank as the air already pumped in resists the air coming in and when you release the air, pressure drops. Before I was educated as a marine engineer I always thought that pumps were rated by pressure to. On shipboard fire fighting pumps the standard procedure is to fully open the suction, overboard and discharge lines and squeeze down on the overboard globe valve until the discharge pressure is raised to 100 psi. Some pumps do provide more back pressure before stalling (or breaking) such as positive displacement pumps (piston type) or non-positive displacement multistage centrifugal pumps. I worked on ships that had boiler feed pumps that had to overcome 1,600 psi to get water into the water/steam drum. It's all the same theory as why hydraulic accumulators work, pump and build a pressure head and release the pressure to actuate and do work. This is what a steam head on an accumulator on a aircraft carrier catapult does to shoot the shuttle down the deck.
I had a lot of trouble with one of my motors in my 101 twin with an erratic needle.
