How does a bicycle pump *head* work? Asking about the pump heads that are pressed on the valve, not screwed. One has to lift a lever before pumping. After pumping, push the lever down before removing pump head. I would like to understand the mechanism of the pump head (not the pump itself). Google didn't help. 

What I wonder about is that the pump head's cylindrical opening is loose on the (cylindrical) valve and then lifting the lever creates a tight seal between the pump head and the valve. How is this seal created? It seems difficult to narrow a cylindrical opening without something wrinkling, except with an iris valve. But an iris valve would require turning, not lever-lifting.
What you have is basically a cylinder of rubber. When you lift the lever, the cylinder is compressed on the circular faces. This results in the sides expansion, which will "hug" the valve giving the sealing effect.