closing technique of bunching the material of a fold allowance into a series of pleats around a convex curve of a pattern piece
Pleats are often hammered down flat, as with a folding hammer, or trimmed flush, as with a long knife.
Theory
Along a convex curve, adding fold allowance creates a longer outer edge than the intended finished, folded edge of the pattern piece. When folded back, this outer edge of the fold remains longer than the edge of the final edge. Its flat length must be reduced somehow to make up the difference.