What they mean by "unloaded" is releasing the air pressure from the gun. The Salvimar uses a reversed power regulator with the control valve being an upstream valve instead of the usual downstream valve. If you set the power regulator to low and then release the air via the inlet valve being depressed then the air in the tank forward of the power regulator bulkhead will be trapped in the gun. This will make the power regulator impossible to budge unless you re-pressurize the gun. To let all the air out the power regulator has to be pushed forwards in the gate which is full power and holds the upstream valve open, usually full power is rearwards in the gate on most of the other gun brands. If you look closely at the gate you can see plus and minus signs indicating high and low power respectively. The one-way non-return valve and power regulator valve are combined in the Salvimar gun, in other guns they are two separate items.
"Easy loading" is just a way of taking a number of separate pushes on the spear to load the gun. Each time you get the spear and thus the piston further into the gun the air being compressed in the front tank stays there because the one-way/power regulator valve stops it going back into the inner barrel when the piston is allowed to travel back to the muzzle when you stop pushing on the loading bar. In a gun without a power regulator any failure to latch the piston wastes your energy as the air that had been compressed by your pushing effort all flows back into the inner barrel and you are right back to where you started. The further in that you can push the spear each time in a gun with a power regulator set to low power (which closes the valve in a partitioning bulkhead) the air in the front tank gets ever closer to the cocked gun pressure as the one-way valve action lets air in, but not out, so it is a step-wise procedure, while the air remaining in the inner barrel decreases in pressure as the gas molecules that used to be there are now added to those in the front tank. A low power shot uses only the air in the gun that is not inside the front tank, that lower pressure left in the inner barrel after such a shot is what makes it a low power shot, whereas a high power shot uses all the air by opening the power regulator valve which was keeping the elevated pressure locked up in the front tank. Of course the gun has to be latched before you open the power regulator valve by setting it to full power.