FLIP will 'flip' an image in either vertically (about rows) or horizontally (about columns). FLIP is used, for example, to change the orientation of an image so that it matches the way it is viewed on the sky or on a finding chart, or to get the wavelength dispersion axis running the way you want before using MASH, etc..
A flip in ROWS inverts the image from top to bottom as seen on the television.
A flip in COLUMNS inverts the image from left to right.
You can flip spectra around with FLIP COLS. This is useful because, in those cases in which the dispersion does not run in the desired direction, it is faster to flip the spectrum rather than the image after MASH. It does NOT make any sense to FLIP a wavelength-calibrated spectrum. Examples: