I was tazed once. Fucking hated every second of it. It was only a few seconds but felt like minutes.
They cause muscular contraction. Its totally involuntary. Imagine flexing your muscles as hard as possible.
So, the gun tazers shoot little darts, and theres a small thin wire from the gun to the dart. It shoots two darts and then once in your body the darts send an electric arc to one another. So the further away the darts are, the greater effect.
If one dart goes into your left shoulder and the other goes into your left bicep, it will hurt but probably wont seize you up and drop you on the ground. But if one dart goes into your right calf, and the other goes into your left pec.... you're going to drop, stiff as a board and its going to hurt like hell.