Rava and Abaya argue and Rava holds one cannot be matfis. As Rashi at very top of 20b explains Rava learns the braisah to not be talking about matfis a neder but rather the Ikar Neder with "teli'ah".
The gemorah explains the chidush is the case of Gedalya Ben Achikam. In that explanation the gemorah uses the terminology 'v'hai lav matfis", saying one would think it isn't Matfis and doesn't work, but indeed it does.
The use of the terminology must mean 'teli'ah and not literally matfis. But, why does the gemara choose to use the word 'matfis' to express 'teli'ah', at arguably the worst time and place possible - immediately after stating that Rava doesn't hold of matfis - why cause confusion and possible misunderstanding!?
Daniel Gray, TORONTO
The Gemara at this point has left behind the argument of Rava and Abaye. It is asking a question on Shmuel, who establishes that the Beraisa is discussing a situation of "Noder u'Va". As Rashi explains, we are asking that if the Beraisa is discussing "Noder u'Va" what is the point of mentioning that the day happened to be the Yahrzeit of the person's father? This question of course applies equally to Abaye or Rava.
The answer of the Gemara is that the Beraisa is teaching that this is called Hatfasah (according to Abaye - or Teliyah according to Rava) from a Davar ha'Nadur and not a Davar ha'Asur. This is equally true for both Abaye and Rava, but the Gemara has to choose a word to use - either Matfis or Toleh. It uses the word 'Matfis', assuming that we will make the proper substitution according to Rava.
Best wishes,
M. Kornfeld