Theory is that you shorten the rods at the back and thus lower the back of the car. And then put more hydro fluid in to raise the whole lot back up. If you shorten the rods without adding fluid, then yes, the back will sit lower (in theory, bear in mind the hydrospastic factor, it could do anything).

It wouldn't hurt to evacuate the whole system before you pump it up, just in case there is air in there.