the simple answer is “its attracted to the bottom of the rail”
How exactly that physically works requires knowing a lot more about the design than we have, but if I were designing one, I would put a steel band in the bottom of an aluminum (or stainless steel) band. magnets are attracted to the steel band, but not aluminum or stainless. So the magnet feels a pull towards the steel band, and will want to minimize its distance to it. but since there is a curved bit of metal in the way, it has to rotate around it, thus orienting correctly.
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