Short selling

Two quite different mechanisms carry the same name. In the cash market the seller borrows the security from a holder, sells it, and later buys it back to return it, paying a borrow fee for the period and passing any dividend to the lender as a manufactured payment. In the derivative market nothing is borrowed at all: the counterparty writes a contract whose selling side gains as the reference price falls, which is how a short contract for difference works.

The cash market version is regulated in ways the derivative version inherits indirectly. Selling without having arranged the borrow, known as naked short selling, is restricted or prohibited in many jurisdictions, and rules commonly require a locate before the sale. Large short positions must be disclosed above stated thresholds in several markets, and regulators have imposed temporary bans on shorting named securities or whole sectors during stress. A ban changes what the counterparty to a derivative can hedge, so it reaches the derivative market whether or not it names it.

Two risks are specific to the activity. The adverse move has no ceiling, since a price can rise without limit, and the borrow itself can fail: a lender can recall the security, and the fee to borrow a heavily shorted name can rise sharply, both of which can force a position to be closed at a moment not of the holder's choosing. Whether short selling stabilises markets by correcting overpricing or destabilises them by accelerating declines is an old academic argument with published evidence on both sides, and it is the argument behind every temporary ban.