Plant
Introduce it early and say nothing about it; leave it alone until a later event makes it matter.
Chekhov's gun: hung in Act One, fired in Act Three.
Seen in Orson Welles, Citizen Kane, 1941
Try it on your work
Hide or embed one deliberate thing where future work will uncover it (a note in the materials, a part in the stock, a marker in the files), addressed to whoever finds it. Place it, record nothing. Forgetting it is part of the operation.
Write one sentence to yourself for a specific future moment ("read when the project stalls"), seal it, and set the trigger. The act of writing it tells you what you already expect.
Insert one provision into your current plan that has no present use and one specific future one: a spare interface, an empty slot, a reversible join. It should cost almost nothing. Note the date; check it in three months.
How it has been used
Introduce something whose significance arrives later: the gun on the wall in act one, the seed under the path, the throwaway line that pays off in the last chapter. The move forbids announcing it: a plant that is noticed as a plant at the time of planting is just a promise.
Where it leads
Grouped under Give me another way in