& and \\. This is done with the
excursion command
\save t \restore
|
where t should be a target in one of the formats described in sections 2.5-2.6. t can do any kind of typesetting desired, for example,
\xymatrix{
A \ar@{-}[dr]
&{}\save[]+<3cm,0cm>*\txt<8pc>{%
This is a very big commentary
but it does not otherwise affect
the diagram.}
\ar[l] \ar[d] \restore \\
& B \ar@{-}[r] & C \ar@{-}[r] & D
}
will typeset

It illustrates how a "down" arrow does not necessarily have to point particularly straight down - in this case because it is based in the displaced pseudo entry.