We investigate the martensitic transition of melt-spun Ni-Mn-Ga ribbons, during which an abrupt release of inteal stress occurs. The release results in the appearance of two peaks on the curve of field-induced strains versus temperature. During and after the transformation, a large lag of field-induced strains was observed, indicating a competition between the stress energy and Zeemann energy. The lag leads to an unusual relation of the strains to the inducing field.