My Physics classes are rusty, but if you've 400GPa across a 1mm^2 surface, and the back of the two opposing diamonds is e.g. 1cm^2, then you've a straight pressure reduction then and there -- not unlike what might happen with a lever or a pulley. So you can then apply a smaller force on the outside, allowing you to use a less sturdy/expensive cristal or metal (or liquid, apparently [0]).