Sciatic - Gid HaNaSheh
The episode of YaAkov battling the angel is a very well remembered image. As is the conclusion of that wrestling match where the angel strikes Jacob injuring him and leaving him limping. The verse declares, and therefore the descendants of YaAkov do not eat the sciatic nerve until this day.

RaMBaM in his commentary on the Mishneh is at pains to clarify that this is not Halachically true. We keep Gd's commandments because Gd commanded them, not because they commemorate historic events.

The seventh chapter of Chullin is named Gid HaNasheh which is the sciatic nerve that we are not permitted to eat. Only the inner Gid that runs along the bone is prohibited by Torah Law. The outer Gid that runs along the meat, is prohibited by our Sages as are the sub-branches and also the Myelin sheathing that surrounds them.

The Gid HaNasheh is prohibited in all Kosher mammals [unlike the prohibited kidney fats, Cheilev, which are only prohibited in BeHeimah not Chayos] but not poultry, since the thigh of a bird is shaped differently and therefore has no “Kaf”, no concave shaped component.

Another differentiating feature relates to the Mitzvah of KiSuy HaDam, covering the blood of the Shechted animal, which applies only to a Chaya but not a BeHeimah. Shulchan Aruch (YDeah 28:4) rules a buffalo being a BeHeimah does not require KiSuy HaDam.

The Rama however rules that it is a Safek, we cannot be certain that buffalo is a BeHeiMah it might be a Chaya. Deer and giraffe are definitely Chayos [and yes we DO KNOW where to Shecht the giraffe - but they probably taste terrible]

Accordingly, the buffalo's kidney fats, Cheilev are forbidden, either as definite Cheilev or Safek Cheilev.
It requires KiSuy HaDam, without a Beracha, because it might be a Chaya.
Other beasts that fall into this area of doubt include, the yak, the African Cape buffalo, the American and European bison. 

Nikkur or Trabering
The Cheilev and the Gid HaNoSheh must be removed, or, as is more usually the case these days, the entire hind-quarter section is not available as Kosher. It is sold as non-Kosher.

Removing these components, as well as various glands and veins is known as Nikkur [performed by a Menakker] or in Yiddish, “Trabering,” apparently derived from the Aramaic word for Cheilev, Tarba.

So tiresome and painstaking in this work that Rebbi Meir rules that butchers cannot be trusted to remove the Cheilev and Gid [Chullin93b] A reliable person must verify that the prohibited components were correctly excised. Although we do not Pasken according to R Meir, the custom in many places the follows this stringency.

Not Using Hind-Quarter Cuts
Since removing the forbidden components is such a difficult and dangerous procedure, the contemporary custom developed in many places to take Kosher meat exclusively from the forequarters.

Unsurprisingly, there is no clear agreement about the tradition defining the boundary between the fore and hind quarters. Many, perhaps the vast majority, consider below the twelfth rib to be hindquarters. The cut extends to the navel.

The Noda BiYehudah (YD 2:31, d. 1793) argues that some of the fats located between the 11th and the 12th rib are Cheilev and notes that this was the old custom in Pargue i.e. pre-dating his arrival as Rav.

On the  other hand, the Chasam Sofer (YD 68, d. 1839) argues that fats above the thirteenth rib are removed only as a custom. 

Clearly, neither the Noda BiYehudah nor the Chasam Sofer was concerned about using the hindquarters, as long as Menakerim cleansed it.

Historic Practices
In central Europe starting in the late 1700's, in those places where the ancient tradition of home Shechita, Kashering and Nikkur was beginning to wane, with that dirty work being performed by the butchers and overseen by the local Rabbi, we begin to witness a preference to avoid Trabering the hindquarters. This was driven both by the Rabbis, who would find it much easier to avoid the effort and risk and also the butchers, who found it more economical and less tiresome to Shecht more animals and sell the hind-quarters to the local non-Jews. [in those days there was little emphasis of the preference of eating hin-quarter cuts]

The MaHarShal
Much earlier, the Maharshal [d. 1573] was distressed that many/some Menakerim had heavy workloads or were paid on the volume of work they completed, which pushed them to be less than as careful as they may have wished to be. They were too easily bullied by the businessmen butchers to work quickly and consequently it was not unusual to find residual Cheilev in the meat. The MaHarShal emphasises that the Menakerim must be truly Gd fearing. It is not enough to have someone who just has the skills.

Perhaps the widespread custom to have a second supervisor check the Nikkur was established for all butchers in order to avoid singling out any particular butcher or Menakker. see Yam Shel Shelomoh, Chullin 1:2, 7:19; BaEr HeiTeiv, YD 65:6)

Is NOT Performing Nikkur a Binding Custom?
The Polish parliament in 1936, under the deceptive flag of preventing cruelty to animals, and respecting the rights of their Jewish citizens, granted permission to eat Kosher meat exclusively to Jews. Non-Jews were not permitted to eat Kosher meat. Whilst this granted the Jews access to Kosher meat, for all intents and purposes it made producing Kosher meat a financially impossible task because there was no opportunity to sell the Tereifos and the hindquarters but for pet food, which made it impossibly expensive.

In those days the number of Tereifos was not that great, the problem was however, that their custom was not to cleanse the hind-quarters and a custom is binding like a vow.

Rav Chayim Ozer Grodzenski permitted reintroducing the practice of Trabering the hindquarters since the practice of not Trabering was driven by pragmatics and economics.

He also felt that it was an obligation to permit the hind-quarter meat, because the decree was essentially an attack upon Yidden and Yiddishkeit. [Achiezer 3:84]

Rav Chayim Ozer persuaded many Chassidishe Rebbes including the Bobover and Lubavitcher Rebbes to support his Pesak.

Rav Moshe Feinstein also ruled that provided a proficient Menaker performed the cleansing there was no reason to prohibit this activity or to avoid eating the meat. Rav Moshe basically repeats R Chaim Ozer's reasoning; not using the hindquarters was driven by economics not by Halacha (IgMoshe YD 2:42)

However, Rav Moshe Sternbuch, Av Beis Din of the Eidah HaChareidis in Yerushalayim, understands that Trabering the hindquarters is a Minhag and may not be changed. (Teshuvos VeHanhagos 1:418, 419)

Rav Shamshon Raphael Hirsch [d. 1888] wrote to Rav Yissochor Berish Bernstein, the Av Beis Din and Rosh Yeshiva of the Hague, that one should maintain the ancient custom of not Trabering (Shemesh Marpei #34)

Appetite for Expensive Cuts
There is a growing sector of Frum Yidden who are prepared to pay significantly more for the perceived pleasure [see R Moshe Fienstien's comments] of consuming hind-quarter cuts. Businessmen have therefore developed this opportunity and are supplying such cuts with suitable Kosher certification.

The OU
The OU have a policy [reported by Rav Y Kaganoff] to authorise removal of the Gid but not the Cheilev. In practical terms - they certify deer which has no Issur Cheilev but only the Issur of Gid. [which is only slightly astonishing because, asking those who have the practical knowledge of Nikkur through a family Mesorah, usually of Moroccan and Teimani origin, many of whom still perform Nikkur at home, it is clear that removing the Gid is more fiddly than removing Cheilev]
​Bison (American buffalo) however, are not Trabered because we are not certain that it is a Chaya. Its hind-quarters are sold as non-Kosher.
Our interaction with Gd and Life, is predicated upon the premise that everything is permitted, other than that which is specifically prohibited.

​Our Sages take a very dim view of those who look for unnecessary stringencies and seek to reduce the enjoyment of those things that Gd created for us to enjoy.

This is fake fear of HKBH.

It is deceptive, both of others and of oneself. 

The Soul, the Neshama is what we today describe as one's inner consciousness. The masters of esoteric wisdom categorise five broad levels with the highest being YeChiDaA. This name reflects the status of that level as being alone. Certainly it is great to be in a crowd, to be with friends and to be with a team. These all contribute to ones self awareness and feed ones drive, ambition and energy levels.

However, in some measure, these are not ones own but belong and attributed to the group.

Let's imagine you are travelling to the Holy Land to join your Rosh Yeshivah or your Rebbe, your spiritual guide and energiser. However, your flight experiences mechanical problems and you end up over the high holidays in an expensive hotel many miles from the nearest Jew.

You have all your requirements, Kosher food, a Machzor, your Tallis, a Shofar and you know how to blow it. You might spend the day immersed in Tefillah or watching movies or basking in the sun reading a book.

No one will know but you and HKBH.

That is the identity of the YeCHiDaA; you are alone with HKBH - what does your day look like?

​If you had a chicken to Kasher, would you bother splitting it when no one would know the difference?