I have not used this, but am including it for completeness. While this is used everywhere in Lenya, I learned about it after I created workarounds with more flexibility.
The full documentation is at JPath Logic Sheet on the Cocoon website.
Add the "jpath" namespace to your XSP:
The two useful uses are the ContinuationID and form fields:
String continuation = <jpath:continuation/>;
String myField = <jpath:value-of select="myfieldname"/>;