In the following extract of a lecture, Patrick McCloskey, Doctor of Scholastic Philosophy, explains the difference between “first act” and “second act” in his discussion regarding “potency” and “act”.
“First act” is ”being”, whereas “second act” is “doing”. What Pope Benedict XVI renounced is the “doing” of pope but not the “being” of pope. Therefore, he remains pope. Furthermore, the “powers” of the papacy belong to “first act”. The “operation” of those powers belong to “second act”. As an example, a baby in his mother’s womb is fully man and therefore has the power of reasoning. However, the operation of the power of reasoning will not be actuated until he attains the age of reason. Pope Benedict renounced the operation of the papal powers but not the powers themselves, which he ontologically could not do even if he intended to do unless he renounces the munus (being) of pope.
Here is a pictorial of the division.