I think you cannot achieve more with it than with a ordinary Xpath
expression.
Not so, it takes a string (eg an Xpath expression in a source file
and evaluates it as an xpath.
consider an input doc of the form
<x>
/a/b/c[2]
</x>
using standard XSLT you'd have a hard job to take that document and
evaluate the xpath, but evaluate(x) would do exactly that.
David
XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list