Demonstration F1-11: Hydraulic Press is a popular and dramatic way of illustrating Pascal’s Law. See it in action in this new video starring engineering student Sarah Hall:

Pascal’s Law states that, given an incompressible fluid, any change in pressure at one point in the fluid is transmitted throughout that fluid. In a closed container, like this hydraulic cylinder, that pressure exerts a force perpendicular to the walls of the container. The force is dependent on the pressure and on the area. Here, that force is then transmitted up into a wooden board, breaking it.

hydraulic cylinder diagram - based on public domain illustration by Olivier Cleynen

 You can see this illustrated in this EduMedia animation:

Experiment with it yourself and see how the force vectors change with area in this simulation by Seng Kwang: