This photo shows the system at another picking machine. You will note there is only one contiuous chain which means only one trailer can be unloaded at a time. Instead of hooks for the bines there are containers with spring loaded clamps. A lever releases the clamp, the end of the bine is put in and on releasing the lever, the bine is firmly held.
At the front of the trailer the metal pole slots for the bines can be seen.