Just pull it up and turn it and it sits in position. If you pull it out and look at it you'll see the "mechanism"(:p) and how it works. Incredibly simply and incredibly effective.
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The Series 1's also have this feature, but it certainly is easier pouring fuel from a jerry can on a S2/2A/3/110 as your not working around the seatbase which is flipped up and the steering wheel, and as John stated I don't think Disco/Range Rover have it, cheers Dennis