We’ve all used pump dispensers, and it’s rather counterintuitive that you press down on the top and liquid flows up. 

So how does it work? Well, when you press the top, it pushed this tube, which moves the piston down forcing the liquid in the housing to go either up or down. However, it can’t go down because there’s a glass ball, a spring and a funnel, which act as a one-way valve ensuring that liquid flows in only one direction.

Thus, as the piston is pressed down, the liquid goes UP through these holes through the center of the tube and out the top.
Once you release the top, the spring applies a force to the button of the tube, Pushing the piston back up and pulling liquid back into the housing.
Because of the design of the tube and piston, when the piston moves up, these holes in the tube get blocked by the interior walls of the piston and thus the liquid that refills the space.
Can only come up from the bottle via dip tube. The volume in the housing is now replenished and ready to pumped out again.