Is there a more complete explanation? They state that this method fails for the quadratrix, but the chosen vectors drawn in the picture in the video seem clearl

Is there a more complete explanation? They state that this method fails for the quadratrix, but the chosen vectors drawn in the picture in the video seem clearly nonsensical, so it's not clear to me precisely what procedure was being followed. Edit: I don't think the video leaves itself enough time to do a good job covering this point. But there's a clearer description of the history at Wolfson (2001) "The Crooked Made Straight: Roberval and Newton on Tangents" AMM 108(3): 206–216 https://jstor.org/stable/2695381 https://jstor.org/stable/2695381