On the Kv arm there's a hydraulically operated catch in the end, pull the spool back to release it, then, as its on the turn over circuit, when you push the lever forward at the headland to turn the plough over it automatically pushes the catch down again, Lemken arms work on a similar set up. I can get you some pictures of the set up Monday if you like
Or if your press arm is old and worn and lets go of the press going across the field....also if the press is long overdue an upgrade due to worn arms that won't pick up easily...........I refer to my current press as "Spawn of Satan" due to how damned tempermental it is. I won't even repeat what I called it earlier in an outburst when it was playing up :o :D