Sortable: beforeStop

I can't seem to get the beforeStop callback working; if I do this for
example:
var conf = {
start:function() { alert("it started"); }
}
Then I get the alert when the sorting starts. If however I do this:
var conf = {
beforeStop:function() { alert("about to stop"); }
}
I never see the alert at all. Do any other specific properties need to
be used in conjunction with beforeStop?