The gemara cites a mishna that permits a Nazir to wash his hair with something that may cause hair to be pulled out which would be a violation of his nezirus, since it is a davar sh'eino miskavein. The gemara assumes that this Mishna was authored by R. Shimon who permits a davar she'eino miskavein, otherwise it would be forbidden. Yet, the Mishna concludes that he may not comb his hair. Rashi explains that the reason he can't comb his hair is because it will inevitably pull out hair and even R. Shimon admits that if it is a p'sik reisha it is forbidden. Rashi repeats this point later on 81b more clearly. Tosafos disagrees and says that the reason a Nazir cannot comb his hair, the gemara in Nazir 42a explains is because he intends to pull out the loose hairs. Unlike Rashi who considers it a davar sh'eino miskavein, just that it is a psik reisha and therefore forbidden, Tosafos considers it a davar hamiskavein since he intends to pull out the loose hairs. [The Ritva tries to bring Rashi and Tosafos together by saying that it is not truly a davar hamiskavein, but since it is a pesik reisha, it is like he is intending to do it and qualifies as a davar hamiskavein]. To explain how Rashi fits with the gemara in Nazir, the Keren Orah writes that by combing his intent is not to pull out hairs but to separate the clumps of hairs which is a pesik reisha that it will pull hairs out. Therefore, although the gemara focuses on kavana, the real problem is because it is a pesik reisha that it will pull out hairs.
The Milo Ha'roim (back of vilna gemara) suggests that the machlokes may be dependent on the machlokes between the Terumas HaDeshen and Magen Avrohom (314:5) whether a pesik reisha is permitted on an issur d'rabonon. Being that pulling out hairs would presumably only be a Rabbinic violation of gozeiz (shearing), Rashi may hold like the Magen Avrohom that a pesik reisha on a d'rabonon is prohibited so that would be enough to explain why combing would not be allowed, whereas Tosafos would hold like the Terumas Hadeshen that is is permitted and therefore would not be enough to explain why it is forbidden, therefore they need to say it is because it's a davar hamiskavein. The discussion is based on the Shulchan Aruch who permits the removal of a knife from a barrel in a way where it will inevitably cause the hole to widen. The Shulchan Aruch based on Terumas Hadeshen permits it even if it's a pesik reisha since the making of the hole is only an issur d'rabonon, whereas the Rama says that it is only permitted if it is not a pesik reisha. The M.B. (11) cites many achronim who side with the Magen Avrohom and hold that a pesik reisha is forbidden even on an issur d'rabonon.
Even those who forbid a pesik reisha on an issur d'rabonon, may permit it to be done by a goy. The Mishna Berura (253:51) forbids one to remove a pot from coals that surround it when it will inevitably stir up the coals, extinguishing the upper ones and lighting the lower ones. Even though some forbid it to even be done by a goy, the M.B. says that for a shabbos need one may rely on those who are lenient. The Sha'ar HaTziyun (43) explains that it is pesik reisha that one doesn't really care about on an issur which is only d'rabonon, therefore it can be done by a goy. Even the Magen Avrohom would permit this. The implication is that a pesik reisha on an issur d'oraysa cannot be done by a goy. However, the M.B. (253:99) permits one to allow a goy to put a pot near heater even though it will cook when he later lights the heater to warm up the house. The rationale is that we permit amirah l'nachri on a pesik reisha, implying that even a pseik reisha that one actually cares about and wants, and even on an issur d'oraysa, we permit it to be done by a goy. It is difficult to reconcile these two pesakim with one another, unless the M.B. is relying on the opinions who don't consider reheating a liquid to be an issur d'oraysa so that it would be a pesik reisha on a d'rabonon (which the terumas hadeshen even permits to be done by a Jew).