How can I make ragdolls fly while flailing their arms and legs around?

So, I recently saw the video "Heavy and Scout think they are birds".
When I saw the bit at the beginning where Heavy is flying while flailing his arms and legs around, I was either doing:

a. Dying on the floor;
b. Wondering how I can make him fly in-game;

For 99.9% of the time, I was doing B.

Okay, so, I already know how to make his arms and legs flail around, but I can't seem to make him fly straight.

So, I can only seek help from you wonderful (but sometimes malicious) forum people to find the answer to:

How can I make a ragdoll fly in a straight line while making their arms and legs flail about?

How can I make ragdolls fly while flailing their arms and legs around?