1) Set up the avatar skeleton bones in a manner that has a chain considerably stretched out (Usually happens on Wrist -> Pinky3 the most)
2) Create a mesh, rig each vertex to at least some bone in it
3) Export the mesh, upload with rigs and joint positions enabled
4) Set the mesh to Animated Mesh (Animesh). If necessary, wear it, preferably on Pelvis, but it usually happens even on the ground.
4) Confirmed to happen in the latest version of the official Second Life viewer and Firestorm. This DID NOT happen a few years ago.
I can provide files (mesh .dae and animation .anim alongside a script) to recreate the exact scene in the video if desired. The feedback hub does not let me upload these files directly.