4 As Supreme Pastor of the Church, the Supreme Pontiff can always exercise his power at will, as his very office demands. Though it is always in existence, the College is not as a result permanently engaged in strictly collegial activity; the Church's Tradition makes this clear. In other words, the College is not always "fully active [in actu pleno]"; rather, it acts as a college in the strict sense only from time to time and only with the consent of its head. The phrase "with the consent of its head" is used to avoid the idea of dependence on some kind of outsider; the term "consent" suggests rather communion between the head and the members, and implies the need for an act which belongs properly to the competence of the head. This is explicitly affirmed in n. 22, 12, and is explained at the end of that section. The word "only" takes in all cases. It is evident from this that the norms approved by the supreme authority must always be observed. Cf. Modus 84. It is clear throughout that it is a question of the bishops acting in conjunction with their head, never of the bishops acting independently of the Pope. In the latter instance, without the action of the head, the bishops are not able to act as a College: this is clear from the concept of "College." This hierarchical communion of all the bishops with the Supreme Pontiff is certainly firmly established in Tradition. N.B. Without hierarchical communion the ontologico-sacramental function [munus], which is to be distinguished from the juridico-canonical aspect, cannot be exercised. However, the Commission has decided that it should not enter into question of liceity and validity. These questions are left to theologians to discuss—specifically the question of the power exercised de facto among the separated Eastern Churches, about which there are various explanations."
Source: Lumen Gentium (Vatican.va)