For a while now there has been a chumra that animals should only be slaughtered when they are upside down so that the weight of the animal does not push down on the knife.
This has led to methods like hoist and shackle which have come under fire from animal rights groups.
In the latest issue of Mesorah, R' Yisroel Belsky writes the following about this practice:
שלא הוזכר אף פעם בש"ס וראשונים ואחרונים ופוסקים
it is not mentioned in Shas, Rishonim, or Poskim.
This is all based on a diyuk in a Shach which he says is completely unfounded. He (as well as R' Genack in the name of RYBS) points out that the Rambam (הל' שחיטה ב:ז) makes absolutely no distinction between if the animal is standing or not.
כיצד שוחטין: מותח הצוואר, ומוליך הסכין ומביאה עד ששוחט; בין שהייתה הבהמה רבוצה בין שהייתה עומדת, ואחז בעורפה והסכין בידו מלמטה ושחט--הרי זו כשרה
I am absolutely stunned by this. This practice of only shechting when the animal is on it's back has become very widespread and has led to many problems. To find out that it is not even a legitimate chumra is very disturbing.
R' Belsky concludes with the following
ואני רוצה להדגיש כי צער גדול הוא ובלבול המוחים להמציא דעות
חדשות שלא הוזכרו בש"ס וראשונים על פי איזה דקדוק הלשון
לכן ישתתק הדבר ולא יזכר עוד
I want to emphasize that it is very painful and misleading to make up new opinions that are not mentioned in Shash and Rishonim based on some diyuk
therefore we should be silent about this and not mention it anymore