Restricting transfer port, was how speed was managed in the old days, it is pretty much a disk with several different size holes you can turn and so to some degree restrict the hole size of the transfer port and so also how fast / much air pass thru it.
My 2012 FX Cyclone have such a measure, with 3 settings, where i assume high = 0 restriction, and MED and LOW are then smaller and smaller hole. to restrict airflow.
But today generally the same is achieved by regulating the pressure of the air you shoot with, this air is then held in a smaller volume in front of the valve you open by shooting, this smaller volume is called a Plenum.
Your hammer spring ( adjustable ) it is turn affect how long the valve is open and air rush in behind your pellet, needless the valve have to close before the pellet leave the barrel or you are just wasting air, and it risk destabilizing the pellet as it leave the barrel.
The hammer that hit the valve to open the pressure it is depending on the pressure on the other sice of the valve ( plenum pressure )
So the hammer work against the pressure in the plenum.
So you might see if you turn REG pressure up that your speed drop

, but that is then BECUZ the hammer do not have enough power to crack the valve open or just very little, this i believe are called valve lock.
So you might have to up your hammer spring force when you up the pressure you are shooting with.
Length of barrel have a say here, for instance in my 700 mm long barrel on the maverick i can drop way lower than the 90 BAR it came with from the factory, 70 BAR or so and i can still reach 960 FPS with a 13 grain .177 pellet or slug.