从其他点定义点

Question

有人能解释一下 tf 是怎样到这里来的吗？为什么红色和蓝色不在同一位置？

\begin{tikzpicture}
\draw (0,0) arc (260:280:20);
\draw (6.946,0) coordinate (A1);
\draw (6.946,-2) coordinate (B1);
\draw[rotate around={10:(A1)}, red] (A1) -- (6.946,-2) coordinate (B2);
\draw[red] (B2) node {$\bullet$};
\draw[rotate around={10:(A1)}, blue] (A1) -- (B1) coordinate (B2);
\draw[blue] (B2) node {$\bullet$};
\end{tikzpicture}

tikz 图片是什么样子的

我很困惑