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

  FT_TRANSFORM_GEOMETRY applies a homogeneous coordinate transformation to a
  structure with geometric information, for example a volume conduction model for the
  head, gradiometer of electrode structure containing EEG or MEG sensor positions and
  MEG coil orientations, a head shape or a source model.
 
  The units in which the transformation matrix is expressed are assumed to be the
  same units as the units in which the geometric object is expressed. Depending on
  the input object, the homogeneous transformation matrix should be limited to a
  rigid-body translation plus rotation (MEG-gradiometer array), or to a rigid-body
  translation plus rotation plus a global rescaling (volume conductor geometry).
 
  Use as
    output = ft_transform_geometry(transform, input)
  where transform should be a 4x4 homogenous transformation matrix and the input data
  structure can be any of the FieldTrip data structures that describes geometrical
  data.
 
  See also FT_WARP_APPLY, FT_HEADCOORDINATES, FT_TRANSFORM_SENS, FT_TRANSFORM_HEADSHAPE,
  FT_TRANSFORM_VOL