https://www.seedsavers.org/site/pdf/corn-hp.pdf
Hand-pollination is a technique used by seed savers to ensure that plants produce seed that is true-to-type and that flowers are not contaminated by the pollen from another variety.
The process varies among species, but with plants that produce unisexual flowers like corn, the uncontaminated pollen from male flowers is transported to the unpollinated stigma of female flowers. Once the pollen is transferred, the female blossom is again covered to prevent contamination by any other pollen.