@<>s: it specifies how far "sideways" the arrow should be
moved, e.g.,
\xymatrix{
A \ar@<1ex>[dr]^a_{.} \\
& B \ar@<1ex>[ul]^b \ar@<1ex>[r]^c
& C \ar@<1ex>[l]^d_{.} }
will typeset

A positive distance will slide the arrow in the "^-direction," e.g.,
the two arrows above are slid in the direction of the labels a,
b, c, and d, respectively; a negative distance in the
"_-direction." The distance <@1ex> is often appropriate since it
corresponds roughly to the height of letters like "x," in the type size
being used.