This video shows the process:
The short version is that there's a ring that expands on the inside. I think that the machine rotates once per cycle because the ring is segmented, so can't press evenly in a single stroke.
Fascinating piece of machinery but it does seem to epitomize the comment by an old farmer. "I can get any amount of people to help me on the farm, so long as I buy a machine that does 95% of the work".