Dude, I'm pretty sure that is Salafi doctrine because people constantly bicker about whether it's true or not.
The way I understand it, halal meat is currently
meat from a Dhabihah slaughter. And Dhabihah slaughtering is done by slitting the animal's throat and leaving it to bleed out. This is done to prevent the animal from dying before being slaughtered (death by slaughter is exsanguination in this context), which would make it carrion according to the Qur'an, and thus haraam and not halal.
Am I getting it right so far?
Now, the problem with slitting an animal's throat is that it doesn't kill it instantly. You sever the wind pipe, the arteries and all that, but you leave the spine intact. The animal is conscious until the blood loss becomes so great that unconsciousness occurs. And as with any wound, clotting can happen, which leads to ballooning. Ballooning is when a blood clot shuts off a vein. It's the body's way of trying to minimize blood loss. This can happen during exsanguination and while the animal still dies due to, you know, having its head almost cut off, the process is delayed.
According to a report by Compassion In World Farming, ballooning happens in up to sixty percent of the cases of Dhabihah slaughtering.
Now, if some Muslims decide that stunning the animal is okay, good for them, but when you go to the Pakistani deli on the corner and order halal meat, how do you think they slaughtered that animal? Electrocuted it? Gassed it? Or Dhabihah?