Rag Doll rigs in games are usually less than ideal. The limbs tend to bend and behave in unrealistic ways, and yet multiple constraints present a processor hit that most developers are unwilling to accept. The Rag Doll in these demos was set up in Maya using only a hinge constraint that operates on 1 degree of freedom per joint. The rotation limit on the joint is set by the shape of the collision geometry.
Limiting the freedom naturally constrains the motion and the resultant behaviour, while simple, avoids many unrealistic states.
Even in a more complex example, the Rag Doll still looks believable, even though it does not even have the correct degrees of freedom in the spine or arms.
In fact, this simpler motion can often be interpreted as directed or autonomous, creating an illusion of complexity that is entirely in the eye of the beholder.
Start simple and add only enough complexity to achieve a balanced result.