Answer:
It is all about pressure. When you plug upon the nozzle, there is no way for the pressure of the fluid to rise above a certain value, so pushing in is opposed by the fluid pushing back on your finger, it gets hard! However, if you open the valve and allow fluid to squeeze out, then the pressure can be relieved a bit and that allows you to keep the piston moving down. That is applying an unbalanced force that allows the fluid to accelerate from zero velocity inside the barrel to some higher velocity through the nozzle.
[tex]\huge\mathfrak\red{i \: hope \: it \: help}[/tex]
