V4 uses magnets to adjust the figure's mesh in response to posing (to make the breasts move realistically as the collars flex and to correct what would otherwise be deformations). The magnetise poses make the magnets affect the clothing too, so that you don't get poke-through. Magnetising has almost nothing to do with fitting the clothing to morphs (some of the breast morphs use the magnets, so they do carry over).