1. A husband may be Meifer (annul) a vow made by his wife on Shabbos.
2. A Torah scholar may permit a vow on Shabbos only if the vow pertains to Shabbos.
3. One is generally not permitted to measure items on Shabbos.
4. One is allowed to tie a non-permanent knot on Shabbos.
5. This law (#2) applies even if the person could have asked to annul his vow before Shabbos.
A BIT MORE
1. The Gemara inquires as to whether this applies only to vows that are necessary for Shabbos.
2. Since a Torah scholar may permit a vow at any time (as opposed to a husband who may do so only "b'Yom Sham'o" -- "on the day he hears it"), he must wait until after Shabbos to deal with it -- unless the vow's annulment is relevant to Shabbos.
3. However, if he needs to measure an object of Mitzvah (e.g., to ascertain if a Mikvah has the minimum amount of water to be valid), it is permitted.
4. The Mishnah derives this from an incident in which such a knot was tied on Shabbos.
5. The Gemara says that the Rabanan permitted a vow for Rav Zutra, the son of Rebbi Zeira, on Shabbos, even though he had time to inquire about it before Shabbos.