There is no need to pass anything to ffmpeg you can just grab the desired format, in this example, it was the "95" format.
So once you know that it is the 95, you just type:
youtube-dl -f 95 https://www.youtube.com/watch\?v\=6aXR-SL5L2o
that is to say:
youtube-dl -f <format number> <url>
It will begin generating on the working directory a <somename>.<probably mp4>.part which is the partially downloaded file, let it go and just press <Ctrl-C> to stop the capture.
The file will still be named <something>.part, rename it to <whatever>.mp4 and there it is...
The ffmpeg code:
ffmpeg -i $(youtube-dl -f <format number> -g <url>) -copy <file_name>.ts
also worked for me, but sound and video got out of sync, using just youtube-dl seemed to yield a better result although it too uses ffmpeg.
The downside of this approach is that you cannot watch the video while downloading, well you can open yet another FF or Chrome, but it seems that mplayer cannot process the video output till youtube-dl/ffmpeg are running.