Any interpolation method can both upsample and downsample.  Downsampling has the advantage that you have more than one pixel to determine the value of each pixel in the output so it is very accurate.  The only problem is that you can end up with aliasing if you have repeating fine details in the original (moire patterns on roof shingles for example) because that detail is beyond the output resolution and is higher than the "sampling frequency" of your new output resolution.  To prevent that, you have to filter out patterns that will be higher than the chosen output resolution and that is done with a gaussian filter, and that is the reason there is an anti-aliasing slider in Qimage's final print sharpening options: Edit, Preferences, Printing Options.

Regards,
Mike