The 77mm Converter Cap from Optech is held in place via a 77mm metal adapter ring that threads on to your lens. The Converter Cap is permanently attached to the ring and can be "flipped" out of the way with your thumb when you want to take a photo. When you're not shooting, just flip the cap down and it covers your lens.
This unique system provides you with a lens cap that you no longer have to worry about losing while still providing you with a way to add additional filters to your lens. The adapter ring is front threaded so adding a filter is never an issue. Another benefit of the cap is that it will help keep dust and dirt off your lens. It is also designed with an O-ring seal that makes it watertight preventing moisture from seeping in.