Press casting is when you make a one part mould out of, for example, greenstuff. I believe Xander has a video on it, not to sure though. It’s something about greenstuff casting if I recall correctly, so be sure to check it out.
The idea is that you take whatever you are “casting”, lubricate it a bit, press it into your greenstuff lump.
Let it set, remove the part that should be cast and then fill the “mould” up with greenstuff again.
After that piece of GS has set you have made a greenstuff cast of your object.