The Canon 52mm Drop In Circular Polarising Filter is a rear-mounted drop-in filter.
The Canon drop-in circular polariser filter is designed to reduce reflections and glare from glass and water and is often used to increase the contrast and saturation in blue skies.
The filter can be rotated without having to remove the holder from the lens using the ring on the outside of the filter mount.
The change in effects can be seen via the viewfinder.