A snippet lifted from ATContentTypes/content/image.py that shows what it's all about:

    def __bobo_traverse__(self, REQUEST, name):
        """Transparent access to image scales
        """
        if name.startswith('image'):
            field = self.getField('image')
            image = None
            if name == 'image':
                image = field.getScale(self)
            else:
                scalename = name[len('image_'):]
                if scalename in field.getAvailableSizes(self):
                    image = field.getScale(self, scale=scalename)
            if image is not None and not isinstance(image, basestring):
                # image might be None or '' for empty images
                return image

        return ATCTFileContent.__bobo_traverse__(self, REQUEST, name)