\lefteqn{eqn}
prints eqn in display math style, but pretends that it has
zero width. It is typicaly used within an
Eqnarray environment for
displaying long equations that require multiple lines.
A typical use might be
\begin{eqnarray}
\lefteqn{\int_0^\infty some expression =} \\
& & some terms \\
& & \mbox{} + more terms
\end{eqnarray}
Note that the empty \mbox{} is so LaTeX
knows that the following + is a binary operator, and is not
attached to the first of the "more terms" as a unary operator.