IEEE Spectrum article on tasers. Talks in detail about how the electrical signals from the taser make your skeletal muscles lock up, but how they're much less likely to screw with your heartbeat.
I'm somewhat ambivalent about tasers. I think that, forced to choose, I'd much rather be tased than shot. On the other hand, it seems that police officers are increasingly reaching for their taser when harsh language would be more than adequate.
I think what we have here is a case of trying to put a technological band-aid on a problem that's caused by the inherent nature of human beings. You can't stop people from being lazy, or uncaring about other people's pain - not even if those people are police officers. The stats show that fatal shootings go down when police officers have tasers. But I suspect similiar studies would show that tasings go up when police officers have tasers. If you give someone a tool, they're more likely to use it. Shocking.
So, while tasers are an improvement over guns, they're not an instant solution to all of our problems. If only life were that easy...