\smash ¶Synopsis:
\smash{subformula}
Typeset subformula as if its height and depth were zero.
In this example the exponential is so tall that without the
\smash command LaTeX would separate its line from the line
above it, and the uneven line spacing might be unsightly.
To compute the tetration $\smash{2^{2^{2^2}}}$,
evaluate from the top down, as $2^{2^4}=2^{16}=65536$.
(Because of the \smash the printed expression could run into the
line above so you may want to wait until the final version of the
document to make such adjustments.)
This pictures the effect of \smash by using \fbox to
surround the box that LaTeX will put on the line. The
\blackbar command makes a bar extending from 10 points below
the baseline to 20 points above.
\newcommand{\blackbar}{\rule[-10pt]{5pt}{30pt}}
\fbox{\blackbar}
\fbox{\smash{\blackbar}}
The first box that LaTeX places is 20 points high and 10 points deep. But the second box is treated by LaTeX as having zero height and zero depth, despite that the ink printed on the page still extends well above and below the line.
The \smash command appears often in mathematics to adjust the
size of an element that surrounds a subformula. Here the first radical
extends below the baseline while the second lies just on the baseline.
\begin{equation}
\sqrt{\sum_{0\leq k< n} f(k)}
\sqrt{\vphantom{\sum}\smash{\sum_{0\leq k< n}} f(k)}
\end{equation}
Note the use of \vphantom to give the \sqrt command an
argument with the height of the \sum (see \phantom & \vphantom & \hphantom).
While most often used in mathematics, the \smash command can
appear in other contexts. However, it doesn’t change into horizontal
mode. So if it starts a paragraph then you should first put a
\leavevmode, as in the bottom line below.
Text above.
\smash{smashed, no indent} % no paragraph indent
\leavevmode\smash{smashed, with indent} % usual paragraph indent
The package mathtools has operators that provide even finer
control over smashing a subformula box.