Swivel fold
A swivel fold is a move where a flap pivots around a fixed point, dragging a small triangle of paper with it that is flattened by a short squash-like crease.
Fold one edge of the flap along the marked line while holding the pivot point still. As the flap rotates, a small wedge of paper is pulled out of line; flatten that wedge along a second short crease so it lies neatly. Swivel folds are used to narrow points, shape shoulders and hips, and reposition flaps without adding bulk.
Because two creases form at once, it helps to pre-crease the main fold first and move slowly so the wedge settles where the diagram shows. If the wedge buckles, the pivot has moved; open the fold and try again with a firm finger on the pivot. Swivel folds appear mostly in intermediate and advanced animals.