When Eisav’s angel saw that he could not best Yaakov Avinu, he made one last attempt at maiming the Patriarch by striking his gid-hanashe. To commemorate this battle, Yaakov’s descendants are prohibited from consuming the nerves/sinew which are included under the rubric of gid ha’nashe. The commemoration of the miracle of Yaakov’s triumph in battle over the forces of evil seems counterproductive. Issur achilah, prohibition from eating, is a shev v’al taaseh, passive form of celebrating the miracle, the direct opposite of the manner in which we celebrate the many miracles which are part of our continued existence. Horav Moshe Feinstein, zl, explains that while we have the ability, resolution and fortitude to withstand the vicissitudes and challenges of the exile which will undoubtedly generate much-deserved reward, we still pray to Hashem that we should not be challenged by the nisyonos, trials, associated with exile. Indeed, this is why we ask Hashem (Birchos Ha’Shachar), v’al t’vienu… liyidei nisayon, “Do not bring us into… the forces of challenge.” Thus, the commemoration of the miracle is passive: We ask that we not be tested.
The Rosh Yeshivah also derives from here that it is more praiseworthy not to fall prey to prohibition than to perform a mitzvah. Sadly, there are those who complacently perform mitzvos, which has been their lifestyle since early youth. To refrain from executing an aveirah, however, to distance oneself from sin – specifically because this is the ratzon Hashem, the will of G-d – is more laudatory.