Part III - Here we see that the power of governance is not actually obtained by the priest with
Ecclesia supplet.
CCEO cc. 994 and 995 (CIC d. 144)There are two additional cases in which the absolution of sins is by a priest who lacks the faculty are valid: (1) cases of common error (2) cases of doubt of law or fact. Both cases flow from the understanding of the faculty as an exercise of the power of governance. Specifically, the faculty to absolve is generally regarded as an exercise of the executive power of governance as opposed to the legislative or judicial powers of governance. (… judicial power of governance is exercised only in the external forum.) …
As a mater of fact both codes explicitly acknowledge that the norms on executive power of governance do apply,
inter alia, to the faculty to hear confessions (… CIC c. 144.2 which specifically lists the faculty mentioned in CIC. 966, i.e., the faculty to hear confessions obtained either* ipso iure* or by special grant (CIC c. 966.2).) There are two well-recognized cases in which the Church supplies (
Ecclesia supplet) executive power of governance; namely in cases of common error of fact or law and in cases of positive and probable doubt of law or fact. The Legislator recognized that both situations can arise in connection with the absolution of sins because both codes expressly state that in such situations the executive power of governance is supplied in both the internal forum as well as the external forum. Therefore, if these situations do in fact arise in confession, the Church supplies the missing power of governance in order to protect a penitent who confessed in good faith.
From the standpoint of the penitent, the result of the Church supplying the power of governance under these circumstances is the same as any priest having the faculty to absolve in the danger of death, e.g., the penitent is validly absolved. However, in regard to the priest, it is very different in that he does not in fact ever obtain the faculty in these cases. Instead, his invalid absolution is essentially sanated by the Church. (Some commentators characterize this an an* ipso facto* release of the power of orders for the good of souls. See Cuschieri, at pp. 226-227.)
The Sacramental Mysteries of Healing by The Very Reverend Francis J. Marini J.D., J.C.O.D. in Comparative Sacramental Discipline In The CCEO and CIC, Canon Law Society of America, 2003, pp. 120-122.