Muzzle pressure, the pressure the gases behind the bullet have at bullet exit.  With faster burning powders in lower doses the powder has generated all the gas it's going to long before the bullet gets to the muzzle.  So what gas has been generated is constantly expanding with pressure getting lower behind the bullet.  The more powder you use, the more gas you evolve, more = higher pressure at bullet exit with slower powders, if equal chamber pressure loads are used.  More gas = more blast at the muzzle.  This is my simple explanation, maybe parashooter could illustrate a few loads for us.

The SPP210 was a surplus powder once offered by Wideners.  SPP = Swiss Precision Powder.  After digging around I think it was a lot of Vectan Tu2000.  Some Vectan powders are being imported, can't say if it includes Tu2000.  Ask your local supplier, he'll probably get a smile on his face like the owner of the junkyard did when I asked about an ECU unit for my 89 Volvo.  No words needed said.

By the way, my load was super at 100yds, easily holding the 10 ring with 3/5 in the X, but it was not good at 200.  I think because of transitioning below Mach 2.  Shot dispersal was pretty wide.