%%
%% The LaTeX Companion, 3ed
%%
%% Example 2-2-21 on page I-52 in "Formatting headings --- \LaTeX 's internal low-level methods".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3exa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{135.0pt}

  \setcounter{section}{4} % simulate previous sections

%StartShownPreambleCommands
\makeatletter
\renewcommand\subsection{\@startsection
  {subsection}{2}{0mm}%               % name, level, indent
  {-\baselineskip}{0.5\baselineskip}% % beforeskip, afterskip
  {\normalfont\normalsize\itshape}}%  % style
\makeatother
%StopShownPreambleCommands

\begin{document}
\ldots\ some text above.
\subsection{Subsection Heading}
The first paragraph following the redefined subsection
heading \ldots   \par   And a second one (indented).
\end{document}