In computer physics engines, ragdoll physics is a type of procedural animation that is often used as a replacement for traditional static death animations. A ragdoll is a collection of multiple rigid bodies (each of which is ordinarily tied to a bone in our Skeletal Mesh) tied together by joints that restrict their movement. When the player dies, each rigid body collapses to the ground and thanks to its constraints the death looks realistic.
This is the ragdoll of our familiar mannequin, and it actually looks quite dead.
This technique is improving year by year thanks to the new technologies, and there are games that use it in a non-traditional way, such as the FlatOut series, where you can control the dead mannequin while flying, or Rag Doll Kung Fu, where you control one part of the body and the rest follows along.
In Unreal Engine, achieving ragdoll physics is really simple. Any object can be set to answer at the physic stimulations, as long as that object has Physics Asset...