Rashi explains that Avraham had Eliezer take hold his milah when he took his oath to go only to Avraham's family to find a wife for Sarah because a shevua requires taking a cheftza shel mitzvah in hand. Why is the makom milah considered a cheftza shel mitzvah? The milah was done already -- how is it different than holding a person's arm after he has removed his tefillin?
R' Noson Gestetner in his l'Horos Noson suggests that this Rashi supports the view of the Or Zaru'a who understands that the mitzvah of milah is not the act of cutting, but rather the mitzvah is being in the state of not having orlah (the result). The Ohr Zaru'a brings three other examples of this phenomenon, each of which is interesting in their own right (and which I wish I had more time to discuss):
1) Talmud torah -- the mitzvah is not the act of teaching, but rather the mitzvah is to see that your kids become educated. If that means paying tuition, that's your kiyum mitzvah.
2) Building a sukkah -- interesting that he counts this as a mitzvah, but he defines it as having a sukkah, the result, rather than the act of actually building one. Rashi (Makos 8) also seems to hold building a sukkah is a mitzvah, but Rashi refers to even chopping wood as part of the mitzvah, proving that the act of building is the key.
3) Wearing tefillin -- this one was the biggest surpirse to me, but then I remembered that we discussed this idea in the past (see here). Rav Soloveitchik reads the Rambam as holding that there is an additional kiyum in having tefillin on, aside from the mitzvah one gets for the act of donning them.