Commonly know thing is when you click "r" and confirm recording will start. but, well... it work bad. i once accidentaly turned on recording and turned off after 15 minutes, i had to delete over 40 Gb from files.
It says it takes up a lot of space. The recorder records every drawn frame. If you run at 50ish fps, then you are recording 50 pictures a second, which can be a lot of space.
recording is now only available via lua. There's some other improvements too. It still requires a lua script or other external functionality to use it.