The Square Polarizer for the LEE 100mm System slots into the normal filter holder (as illustrated) where it can be rotated to the desired angle.
It is recommended for studio work or when no graduated filters are required. If ND Grads are important the 105mm Rotating Polarizer with the 105mm Accessory Ring is a better choice as it allows independent rotation of the graduated filters and Polarizer..
The Polarizer removes light which has become polarized by bouncing off flat non-metallic surfaces such as glass or water. Filtering this light can eliminate reflections, remove glare, darken blue skies and generally increase saturation.
It is a Circular Effect Polarizer (nothing to do with its shape!) which is ideal for most cameras because it doesn't interfere with their complex metering and auto-focus systems.