You can also hold those keys to play normally at the speed set with [ and ], so you can actually play the video in reverse and slow motion or whatever. Be aware that it's usually very intensive on cpu time (and maybe gpu decoding if applicable) since it has to usually go back a whole keyframe and compute all frames between while doing that, which may result in less than smooth playback on some videos.
Reverse usually works significantly worse, at least with common video codecs that work on key frames and intraframes. Depending on your work flow, codecs that don't operate on keyframes/intraframes will actually provide mpv the capability of playing backwards at full speed (eg: rawvideo, ffv1, magicyuv...).
