%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-5-7 on page II-183 in "Horizontal extensions in standard \LaTeX {}".
%%
%% 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{tlc3examargin}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{135.0pt}

  % Shorten the vertical size of the example ...
  \AtBeginDocument{%
    \setlength\abovedisplayskip{.1\abovedisplayskip}%
    \setlength\belowdisplayskip{.1\belowdisplayskip}%
  }

%StartShownPreambleCommands
\usepackage{amsmath}  % for the \text command
%StopShownPreambleCommands

\begin{document}
\[
  \overbrace {\psi_\delta(t) E_t h}^{\text{explanation}} =
  \underbrace{\psi_\Delta(t) E_t h}_{\iff x > 0}
\]
\end{document}