I doubt the stunning has changed much since I saw it being done. The sheep were kept in a small holding pen outside the plant, then forced or dragged up a ramp into the killing room, which for most of the day had its walls plastered ceiling-high with blood, with dozens of sheep within sight being strung up by their hind legs onto overhead conveyor chains. You can imagine the noise and smell of hundreds of sheep being transported across an area the size of a tennis court to be skinned and disembowelled. Once they got to the top of the ramp they were trapped in a narrow cage-like frame and given a short blast of current to the head. ECT alone doesn't induce unconsciousness in people so I don't see how it could with sheep. Most likely they were simply rendered unable to move by unimaginable fear. Then the slaughtermen would cut their throats - these guys were good at it, one slash went through all the blood vessels in the neck. Halal butchers will do exactly the same; it's the most humane part of the whole process. Once blood circulation to the brain stops, it takes about 18 seconds for a human to lose consciousness, so it must be about the same for sheep.

As far as I can tell, stunning is simply to speed the process for the human operators. It only increases the animal's suffering.