Emit an electric pulse. Our hearts function based on an electric pulse and our nerves function based on similar impulses. A tazer is designed to "disrupt voluntary muscle control" for a temporary period (thus rendering a subject unable to act), but can affect these "muscles" as well. If you have a pre-existing heart condition, you can, in theory, have issues from being tazered, such as a heart attack. There is a wealth of information (And controversy) about Tazers online if you are interested in learning more.