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.
This is about pressure - the air inside of the syringe takes up some volume and exerts pressure on the plunger. If you keep the air inside from escaping by holding the nozzle closed, then the air will get concentrated to higher pressure as you push in the plunger - until the pressure inside the syringe is the same as the pressure you're putting on it with the plunger, at which point you can't push it in anymore, because the air pressure becomes stronger than your finger.