Understanding forest options when customising trees

Question

This is a follow-up to Gonzalo Medina's answer to this question.

The solution offered there includes the following code at the start of the forest environment:

  edge path={
    \noexpand\path[\forestoption{edge}]
      (!u.parent anchor) -- +(0,-13pt) -|   
      (.child anchor)\forestoption{edge label};
  },

How does this code achieve the desired layout? I am unclear how the explanation of the relevant option on page 33 of the manual relates to the resulting diagram. In general, I find forest's documentation rather difficult to follow when it comes to customising tree layout (as opposed to specifying the relationships between nodes etc.) but I would like to understand it since forest seems to be more powerful and flexible in principle than the available alternatives.

For completeness, the full code of Gonzalo Medina's solution is:

\documentclass{article}
\usepackage{forest}

\begin{document}

\begin{forest} 
where n children=0{font=\sffamily}{},
for tree={
  edge path={
    \noexpand\path[\forestoption{edge}]
      (!u.parent anchor) -- +(0,-13pt) -|   
      (.child anchor)\forestoption{edge label};
  },
  l sep=10pt,
} 
[\mbox{}
  [$C1_2$
    [$C1$ [a]]
    [$C2$ [b]]
    [$C3$ [c]]
    [$C4$ [d]]
  ]
  [$C2_2$
    [$C5$ [e]]
    [$C6$ [f]]
    [$C7$ [g]]
    [$C8$ [h]]
  ]
]
\end{forest}

\end{document}

Answer 1

Perhaps the following variation to the example helps to understand the specification:

\documentclass{article}
\usepackage{forest}

\begin{document}

\begin{forest} 
where n children=0{font=\sffamily}{},
for tree={
  draw=cyan,
  line width=0.2pt,
  parent anchor=east,
  child anchor=west,
  edge path={
    \noexpand\path[\forestoption{edge}]
      (!u.parent anchor) -- +(0,-13pt) -|   
      (.child anchor)\forestoption{edge label};
  },
  l sep=10pt,
} 
[root
  [$C1_2$
    [$C1$ [a]]
    [$C2$ [b]]
    [$C3$ [c]]
    [$C4$ [d]]
  ]
  [$C2_2$
    [$C5$ [e]]
    [$C6$ [f]]
    [$C7$ [g]]
    [$C8$ [h]]
  ]
]
\end{forest}

\end{document}

In

\noexpand\path[\forestoption{edge}]
  (!u.parent anchor) -- +(0,-13pt) -|   
  (.child anchor)\forestoption{edge label};

!u refers to the current node; so

!u.parent anchor 

starts a path on the declared parent anchor for the current node; then

-- +(0,-13pt)

causes the path to move downwards 13pt and

-| (.child anchor)

makes the path go horizontally and then vertically until it reaches the specified anchor for the child nodes. \forestoption simply access the values for the option used in its argument.