This might be noob of me, but I can't get the skinny arm model

The updates tell you that it is possible to switch on the model on your Minecraft profile, but surprise, there is nothing in your minecraft account whatsoever that can do that. It only allows you to stretch a skin over the old 4 pixel arm model. I can't get my head straight why I can't toggle it to the new player model. I have tried looking up on the internet how to do it, but there are no tutorials, so this probably means that nobody can do it. How do you flipping get that player model???? Why didn't they give us a button on our account to transfer to it? Is it an easter egg feature, is it a feature only for certain players, or is it a bug like I think it is?