The Ragatchover writes [citation needed] that a positive Mitzvah can have three aspects to it.
- The obligation to do it.
- The continued fulfillment of it
- The prohibition of stopping it
Most Mitzvos have one or two of those aspects, but Circumcision has all three, which we can see from the following examples
- Someone born circumcised has to have a drop of blood taken. This fulfills the obligation "to do it."
- There is a Medrash which says that Dovid HaMelech was standing in the bathhouse [unclothed] and he thought to himself that he has no Mitzvos, then he remembered that he has his Bris, and this comforted him. So just having it is itself part of the Mitzvah
- It is forbidden to have surgery to make it cosmetically look like the circumcision never happened.
That is what I remember from the Ragatchover. The rest of this answer is my own association.
Other Mitzvos can be different. For example, with Lulav, before saying the blessing, the Esrog is left on the table or held upside down so that one does not fulfill the obligation before being able to say the blessing. Once the Lulav is held properly, the blessing can no longer be said, because it really only has the aspect #1, not #2.
Succah, by contrast, the Halacha is that if someone forgot to say the blessing before eating a meal and remembers after he is finished, but still sitting in the Succah, he can say a blessing at that point, because Succah has aspect #2. However, it lacks aspect #3 - there is no prohibition in leaving the Succah afterwards. [The first night of Succos has #1 for sure, I don't know if the rest of the time does or not].
With p'ru ur'vu it lacks #1 (in other words attempts to fulfill it more constitute a Hechser Mitzvah - preparation for a Mitzvah), therefore children conceived before Bar Mitzvah still count. As the Lubavitcher Rebbe once said, it is a Mitzvah you can choose not to do, but it is not a Mitzvah you can choose to do. With regards to a Ger, this is a Machlokes, but it could be that this a question of if the children have to be related to the person or not, rather than a question of if the Mitzvah has an aspect of #1. Arguably it also has #3, in that if someone's children cannot themselves have children or they die before having children, the Mitzvah is unfulfilled, but it is hard to make a practical example of #3 in this case. Perhaps you could bring the fact that castration is forbidden as evidence for #3 in this case.
