"the good is said to be diffusive of its being in the way the end is said to move."

     "bonum dicitur diffusivum sui esse, eo modo quo finis dicitur movere."

     Thomas Aquinas, Summa theologiae I.5.4.ad 2, trans. Ralph McInerny.
     I.e. the good is diffusive of its being qua final (not formal or efficient, and certainly not material) cause.
     Diffundere is to spread by pouring out.  Yet for Aquinas the good is "diffusive" (spreads itself abroad) by drawing in (attracting) "in the way [a final cause] is said to move".