closing technique of cutting slits in the fold allowance of a concave curve of an upper before folding
Contrast pleating.
Theory
Along a concave curve, adding fold allowance creates a shorter outer edge than the intended finished, folded edge of the pattern piece. When folded back, this outer edge of the fold allowance must somehow spread to track the curve of the finished edge.
This can happen by stretching the material, but the material must be able to stretch that much, and any resulting tension in upper piece must not be enough to distort the edge or the surface. Alternatively, the folded edge can be slit so that negative space between the slits takes up the difference in length of the line.