I remember doing it a few years ago, but it seems that I went the 'wrong' way. You may well ask, 'How can there be a right and a wrong way?' Well, the wheel rested on a kind of ledge, and going in one direction you pushed the wheel against a sort of nut and bolt sticking up from the ledge, as a result of which it (the wheel) stayed where it was. However, if you went the other way there was nothing stopping it moving a least a short distance, and dropping an inch or two in the process. When I tried it this did, indeed, move and drop, and I fell on my side onto the sticky-up bolt thingy, acquiring a spectacular and painful bruise.