What is a short position (short selling)
Betting that a stock falls: borrow shares, sell them and buy them back later. If it fell, the difference is profit; if it rose, a loss.
Why it does not appear in the 13F
The 13F only requires long positions. A fund may hold large shorts that do not show up. That is why a hedge fund's 13F shows only part of its bet.
Puts, however, do
Buying a put is another way to bet on a decline and it is reported in the 13F. Michael Burry's puts on Palantir and NVIDIA are a well-known example.
Frequently asked questions
How much can you lose on a short?
In theory, without limit: a stock can rise indefinitely. That is why shorts are handled carefully.