I’m not speaking for Rennner. When I read his response I thought about a pinch rod as being kind of a story stick.
I use a pinch rod to help measure diagonals, especially for picture frames, where I would do multiples. First frame I will measure the diagonals on the inside and get the frame square. Using a straight scrap stick, with one end beveled to a point, I will make a mark and bevel cut the other end to a point and trim so that it fits perfectly in the diagonals of the first frame. Now I use it on the rest to square up each frame.
I may be over explaining. Really helpful if your doing a lot of frames.
-nicky