The opening Mishna on today’s daf teaches that if someone took an oath to perform a mitzva or to neglect a mitzva, the oath has no meaning and the person would not be held liable for a wrongful oath even if he did not fulfill what he said.
In his Commentary to the Mishna, the Rambam explains that if someone swears to neglect to perform a mitzva he will not be held liable because the passage in the Torah regarding such an oath limits it to situations where the person can choose le-hara o leheitiv – that the individual controls whether to perform that particular action or not (see 5:4). When a person swears that he will not perform a mitzva, since he cannot do so, the laws regulating this type of an oath will not apply. It should be noted, however, that the laws of shevuat shav will apply, since he invoked God’s name in vain when taking an oath that he could not keep.
In the case where he swore to fulfill a mitzva, again the laws of a wrongful oath will not apply, since he had no choice but to fulfill the mitzva. In this case, however, he will not be held liable for a shevuat shav, since a person is allowed to make use of oaths or vows in order to encourage himself to fulfill commandments.
The Mishna presents this ruling in contrast with the opinion of Rabbi Yehuda ben Beteira who believes that the laws of wrongful oaths do apply to someone who swears to fulfill a mitzva. His logic is that if a person will be held liable if he does not uphold a voluntary oath that he takes upon himself, how much more will he be responsible if he does not uphold an oath that is taken regarding a mitzva that he is already sworn to fulfill, that is, the commitment taken by every Jewish person on Mount Sinai.