The first power is the material principle of moral acts, the second the formal
In a moral action one can tell apart its matter or substance from its form, which is its moral goodness or badness. Accordingly he calls the first power the material principle of moral actions and the second the formal principle.
The bare act of willing comes from the will, the natural power by which we are able to act well or badly; willing well or willing badly comes from the habits that incline or determine the will.
Latin
XXXII. Cum autem in actione morali discerni possit materia seu substantia actionis a forma, hoc est bonitate vel malitia morali, dicit potentiam primam esse principium materiale actionum moralium, secundam vero principium formale. Quoniam a voluntate, quae est potentia naturalis, per quam bene vel male agere possumus, est ipsum velle: ab habitibus vero voluntatem inclinantibus vel determinantibus, est bene vel male velle.