Funnel
One way in, one at a time; what does not fit stays out.
A teahouse door at knee height.
Seen in Sen no Rikyū, the nijiriguchi, the crawl-in entrance to the small tea room
Try it on your work
Cut a slot 5 cm wide in the lid of a box, and put every piece of material for a project through it, one at a time. What does not fit stays out of the project. Stop when everything has been tried.
Let ideas for the project into your notes only through one line of ten words each. Anything that needs more stays out. Stop at ten ideas.
A meeting hears only whoever holds one object, passed from hand to hand; one speaker at a time, and anything said without it does not count. Stop when the object has gone round twice.
How it has been used
Give the work one narrow way in and make everything pass through it one at a time: the single slot of a submissions box, the door one person wide, the one-line field every idea must fit into. What does not fit stays outside. The move forbids a second entrance.
Where it leads
Grouped under Give me a useful restriction