Note that this reference documentation is identical to the help that is displayed in MATLAB when you type “help ft_preproc_dftfilter”.

  FT_PREPROC_DFTFILTER applies a notch filter to the data to remove the 50Hz
  or 60Hz line noise components. This is done by fitting a sine and cosine
  at the specified frequency to the data and subsequently subtracting the
  estimated components. The longer the data is, the sharper the spectral
  notch will be that is removed from the data.
  Use as
    [filt] = ft_preproc_dftfilter(dat, Fsample, Fline)
    dat        data matrix (Nchans X Ntime)
    Fsample    sampling frequency in Hz
    Fline      line noise frequency
  The line frequency should be specified as a single number.
  If omitted, a European default of 50Hz will be assumed.
  Preferaby the data should have a length that is a multiple of the
  oscillation period of the line noise (i.e. 20ms for 50Hz noise). If the
  data is of different lenght, then only the first N complete periods are
  used to estimate the line noise. The estimate is subtracted from the
  complete data.
  See also PREPROC