Therefore, the children of Israel may not eat the displaced tendon, which is on the socket of the hip until this day, for he touched the socket of Jacob’s hip, in the hip sinew. (Genesis 32:33)
After a kosher animal is properly slaughtered and inspected according to Jewish law, it must have a selection of large blood vessels, prohibited fats (Vayikra 7:25) and the sciatic nerve removed before it can be salted and sold as kosher. This process is called nikkur (porging).
Porging is known to be a very delicate and precise process. Rabbi Yonason Eibeshitz (d. 1764) was known as one an expert on porging and records an incident where a porger came to Prague claiming that the sinews that the Jews have been removing were the wrong ones. Rabbi Eibeshitz was able to demonstrate that the particular sinew referred to by this man was in fact exclusive to male animals and could therefore not possibly be the correct one.
Rabbi Moshe Isserlis (d. 1572) writes that porging can not be learned from books but must be learned through apprenticeship (Yoreh Deah 64:7). This is because there were often local customs which were stricter than the basic law and had to be learned from experience. Nowadays, many refrain from eating the hindquarters altogether due to the difficulty in porging and the ease at which the hind quarters can be sold to non-Jews.
The question arises as to why the Torah commands us to refrain from eating these parts of the hindquarter. Commentating on the above verse Rashbam (d. 1158) explains that we do so as a remembrance to Jacob’s struggle with Esau’s angel who represented the material world (Rashi on Genesis 32:25). Rabbi Samson Raphael Hirsch (d. 1888) explains that the word נשה from גיד הנשה, sciatic nerve seems to relate to the concept of submission (Genesis 32:33). By refraining from these parts of the hindquarter, we symbolically renounce weakness and submission to the earthy physical pleasures of the material world represented by Esau. Although Jacob limped away from the ordeal, he recovered and became known as Israel, the sign of the sole conquering power of God.