My understanding (of Jewish Kosher slaughter, at least) is that it is not the cutting of a concious animal's throat with a sharp knife by a qualified, pious person that is objectionable. Rather it is the shackle and hoisting of the animal where the animal is suspended upside down in the air by one chained leg, before it is cut and the length of time the animal is made to wait and (I'm not sure if this is true) that more than one animal may be suspended and have to wait in line to be cut.
This is also my understanding. While the animal is hoisted and slaughtered it is fully conscious and hangs for at least several minutes. It was outlawed as inhumane by the Humane Slaughter Act of 1958 which required that the animal be rendered unconscious before being hoisted and slaughtered. 10% of animals are still killed this way in the US eventhough other methods of kosher slaughter have been developed. However, it is still the main method of kosher ( and Halal?) slaughter outside the US. A summary of Temple Grandin's findings: "Hanging a 1,000 to 1,200-pound animal upside down by one leg unquestionably causes tremendous suffering. It is common that this method causes bruising, torn flesh, and even broken bones. Furthermore, stress levels can be measured empirically through stress hormone (cortisol) levels. Stress levels for inverted slaughter with devices known as the Weinberg pen (which are less stressful than shackling and hoisting) have yielded the highest average stress ratings ever published (almost 300% higher than cattle killed in upright pens)."http://www.grandin.com/ritual/conservat ... h.law.html
The shackle and hoist method as described above is not always used precisely because of the debate in animal welfare (not rights) among the rabbinate and others. Kosher slaughter permits that the animals be restrained upright (e.g. in a standing pen) until the moment of cutting or in a box-like holding pen that inverts the animal so that the cut may be made downward, and then suspending the animal upsidedown to bleed out.
And even the pens are considered inhumane because, "For that matter, some of the pens now being used also violate those laws. As Dr. Regenstein has pointed out in an e-mail to us, the technology of some pens requires that the animal be turned upside down. "The Facoima pen is at best marginal - and is used by at least one major OU facility in the US - although they have now gone from 180 degrees of rotation to 135, degrees .... The UK has moved to [require] upright kill. The Weinberg pen, which is not as well designed as the Facoima pen and also moves the animal upsidedown, is unacceptable." (from same source as above).
I think in Europe and the US, kosher slaughter is done mainly, if not exclusively, by the restraint method for animal welfare and worker safety. However, in South America, e.g., which is a large, if not the largest, supplier of kosher beef to the US and Israel, the shackle and hoist method is more predominant and is, moreover, endorsed by the Israeli Rabbinate and the US Orthodox Union.
"Now that kosher, humane slaughter using upright pens is both possible and widespread, we find shackling and hoisting to be a violation of Jewish laws forbidding cruelty to animals and requiring that we avoid unnecessary dangers to human life. As the CJLS, then, we rule that shackling and hoisting should be stopped."
"At Grandin Livestock Systems, we believe that the practice of hanging live cattle and calves upside down should be eliminated."