Are your images a consistent size - i.e. do the listings with the feature enabled have larger images uploaded than the listings without it? 

I have it on all my listings, and as far as I know there is no option to enable it by choice, but it is enabled on images above a certain size, as it effectively acts as a zoom / isolated enlarge feature (so if the images are too small, there is no room for them to be enlarged and zoom in on specific points).

The smallest image I have uploaded is 800 pixels by 800 pixels, and it is enabled on those, but I'm not 100% certain that's how it's enabled (just guess based on my own listings and viewing many others that don't have it), or what the smallest image size you can upload is and still have the feature.