Crop HD (1920x1080) video ratio from the center in ffmpeg
Your command is doing fine, and ffprobe will show the resolution as 960x1080
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf57.83.100
  Duration: 00:10:45.14, start: 0.000000, bitrate: 1969 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 960x1080 [SAR 1:1 DAR 8:9], 1838 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc (default)
    Metadata:
      handler_name    : VideoHandler

My suggestion is to go through some of the examples from https://ffmpeg.org/ffmpeg-filters.html#Examples-52 to see if you have any expected result.
Good luck.