Magnetize replaced JCM's (Joint Control Morphs) in V4.  The best example of this is seen when an arm is raised, the breast is pulled up.  With older models if the clothing didn't have the same JCM you would get poke-thru (the top of the raised breast would poke through the clothing).  By magnetizing you are applying the magnet used to raise the breast to the clothing, thus no poke-thru.  As for how you magnetize, I don't really know because I don't have V4.