Sink fold
A sink fold is a move where a point or corner is pushed inward into the model, reversing its creases so the point disappears inside instead of being folded over.
Crease the fold line firmly in both directions first. Then open the layers slightly, push the tip of the point into the model along the crease, and let the surrounding paper reverse around it before flattening. In an open sink the paper can be spread flat while you rearrange it; in a closed sink it cannot be opened, which makes it one of the harder moves in origami.
Sinks go wrong when the pre-crease is weak or when you push before the layers are free to move. Make the crease sharply, open the model as far as it will go, and ease the paper in rather than pressing hard. Diagrams show a sink with a mountain line across the point and a hollow push arrow. It is an intermediate to advanced technique not needed for any model on this site yet.