Hooping.

Hooping is clamping the garment and its backing between the two rings of an embroidery frame so the machine can drive them together under the needle. The fabric should sit smooth and not move when a finger runs across it, but on ordinary knits it must not be stretched: stitches lock the cloth in whatever state it was held, and a knit hooped under tension relaxes into ripples the moment the frame comes off. Hooping too loose is the same fault from the other side, letting the garment shift so colors drift out of registration. Either way the piece is scrap, because embroidery on a knit does not survive being picked out, and hoop burn on pile or coated fabric fails inspection even when the stitching is clean.