This bridge connects Zaaneiland to Zaandam. It was originally built with no way for boats to pass through, so the opening mechanism had to be designed specially to fit the existing bridge. As a result, the blue arm lifts the bridge deck up and out at an interesting angle (towards the camera and to the left).
To request an opening, boats move up close and press a button, then back away. Electric eyes check for things (cars, cats, bicycles, etc.) on the bridge, or a boat in too close, then the bridge automatically opens. It shuts again a minute or so after no boat traffic has been detected.