How would I do this in Gmod?

Well then, Mentlegen, I have seen in quite a few Gmod videos this certain effect that I would really like to be able to do.
A ragdoll looks like it’s standing up, and when a prop like a chair is thrown at it, it falls over.
So it only falls over when something hits it. It would be a great help if somebody could explain how to do this, if you know =)

Thanks very much.