As shown in the diagram, a 180° pulse is applied on the insensitive nucleus simultaneously with the 180° pulse on the proton. This is the population inversion part of the scheme, where a further 90° pulse after a time period on both the sensitive and insensitive nuclei rotate the magnetization onto the z-axis. This has the effect of producing an antiphase alignment of magnetization on the z axis, an important step during which the polarization is transferred from the sensitive nucleus to the insensitive one.
