Hydraulic pressure intensifier

http://www.jetedge.com/content.cfm?fuseaction=dsp_applications_101

consider a ram coupled with a second one by their rods.

The 1st has a piston area bigger than the second. When you fill the first with pressure the force is transmitted via the rods to the second and since the piston area is smaller the pressure will be higher. That's all. Of course the pressure is not only proposrtional to the arearatio. There are friction losses due to the seals but with modern cyliders the efficiency can be quite high. The only poblem one can be confronted with is the possible pressure drop at each stroke end. In order to avoid it the system has to made more compliant by an accumulator.