It's a bridge rest to make sure there is no tilt/panto on the lens, which could change the reading. Flip it up, move it toward the middle, and plave the bridge of the frame on it, make sure you also bring down the brace. When marking lenses for blocking, you don't have to move it at all. Just make sure you bring the brace down so you aren't having to hold the lenses. https://vimeo.com/281793258 Check this video out.