I'm not sure that anyone can, but just that certain large entities can, and it is apparently profitable for them to do so, so they have incentive to keep it in line. If it's selling for less than it's worth, they will "cash out" at the end of the day and pick up the underlying shares at a discount. If it's selling for more than it's worth, they will buy more underlying securities and create new shares that sell for more than they are worth. I'm no expert though so it would be good to look into it more.
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