Yevamot, Chapter Thirteen, Mishnah Thirteen
This mishnah discusses a woman who made a vow not to receive any benefit from her yavam (her brother-in-law). Since sexual intercourse is a benefit, for it is pleasurable, she cannot have yibbum. The question is, is the yavam forced by the courtto perform halitzah for her, or is he merely requested to do so.
If a woman vowed to have no benefit from her yavam:
1) If the vow was made during the husbands lifetime they force him to perform halitzah,
2) [If her vow was made] after the death of her husband, they request of him to perform halitzah.
3) If this was her intention, [even if her vow was made] during the lifetime of her husband, they request of him to perform halitzah.
Section one: If the woman made the vow while her husband was still alive, and her brother-in-law was not yet her yavam, and then her husband dies, the court forces the yavam to perform halitzah. This is because her vow was not made with the intention of subverting yibbum. She may have made her vow just because she really didnt like her brother-in-law or for any other reason.
Section two: If she made the vow after her husband died, it was obviously made in attempt to circumvent yibbum. While the vow is still valid, and the yavam may not have yibbum with her, she is not automatically granted halitzah. Rather the court requests of the yavam to perform halitzah, but does not force him to do so.
Section three: Similarly, if it was clear that she made the vow only to avoid yibbum, even if she made the vow when her husband was alive, the yavam is not forced to perform halitzah. She cannot use this vow as a means to circumvent yibbum. Her intention would have been clear if, for instance, she said that she was doing so in order to avoid yibbum.