Hi,
The AL-KO 1300 is the favourite coupling stabiliser for the Eriba Touring caravans, and we've built up a fair amount of experience with these couplings.
The red wheel is used to clamp the friction pads onto the tow-ball, and has an internal clutch to stop you applying more force than just "nipping" the pads.
The red handle (on the side of the coupling) is then used to apply a fixed torque onto the coupling pads.
When you clean the tow-ball, remember to clean out the inside of the AL-KO 1300 with clean rags (with maybe white spirit or brake cleaner) - crud builds up inside there too
Robert