The Zeeman effect on positronium (Ps) ground state sublevels displays two consequences: shortening of the triplet lifetime ('magnetic quenching') as well as a charge of formation probability for triplet and singlet sublevels. This last effect, which can be observed only for helical positrons, depends on the direction of the positron spin with respect to the applied field. Although only the quenching effect is usually considered, magnetic quenching experiments carried out with spin polarized positrons display a complex and fascinating phenomenology, whose understanding is probably correlated with the positron-medium interactions taking place within the short time interval preceding Ps formation.