The mechanism is recognition. A payoff works because the audience remembers the setup, and remembering it is itself a small pleasure the audience attributes to the story and never to their own memory. This is why payoffs are the cheapest emotional effect available and why over-using them makes a piece feel mechanical.
The ratio matters. Every setup you introduce is a debt, and unpaid debts accumulate as a vague sense that the piece did not add up. Three setups, all paid, is the target. Nine with five landing is the usual failure. Cutting an unpaid setup is almost always an improvement.
The strongest version reverses the meaning. A line that was a joke in the first minute and lands as something else in the last minute is doing real work. A line that merely recurs is decoration.
What is setup and payoff?
Establishing something early and returning to it later with new weight. It produces the sense that an ending was earned, because the audience recognises the element and attributes the satisfaction to the story.
How many setups should a short film have?
Few, and all of them paid. Three paid setups beat nine of which five land, because unpaid setups accumulate into a general feeling that the piece did not add up.