Origami Sink Fold
The origami sink fold is designated with a broad arrow and a mountain-fold line (dash-dot-dot). The arrow indicates a “push in” action while the line shows where the fold will occur.
How to do the Sink Fold:
1) Fold at the indicated line back and forth (mountain and valley folds). Do this a few times to loosen the paper. Note that all 4 creases will be mountain folds when the sink is complete but for now, a back and forth motion is good.
2) Ease open the paper a little (in this example, a waterbomb base) and push the top of the paper down.
3) Continue to push down until the tip of the paper is sunken in between the layers of the paper.
4) Complete the sink fold by compressing the layers together.