European 'jacks' are known (in English) as 'jockey wheels' and have two means of adjustment:
- undoing the clamp on the side allows the whole tube to be slid up and down (where Paul's red handle means he doesn't have to grab the dirty tube);
- turning the handle on the top is done only for fine adjustment.
So for example, unhitching the trailer is usually done by undoing the clamp, allowing the jockey wheel to drop to the ground, tightening the clamp and then winding the handle as the hitch is unlocked.
Forgetting to fully tighten the side clamp (or it rattling undone) and the jockey wheel dropping while driving isn't unknown! Superior jockey wheels have a ribbed shaft so they don't just rely on friction to hold them in place.
And remember that since all European trailers have those old-fashioned rod/cable-operated mechanical brakes
, they all have a handbrake also.