Cattle are rendered unconscious by applying an electric shock of 300 volts and 2 amps to the back of the head, effectively stunning the animal,[10] or by use of a captive bolt pistol to the front of the cow's head (a pneumatic or cartridge-fired captive bolt). Swine can be rendered unconscious by CO2/inert gas stunning. (This step is prohibited under strict application of Halal and Kashrut codes.)
http://en.wikipedia.org/wiki/Slaughterhouse