sortable_element helper -- How do I prevent round trip to server?

I'm using the sortable_element helper method, and for my app I want to
use a javascript snippet update the "position" field without going to
the server and saving the data right away. Other things on the page
could be changing and the re-order needs to be a quick, lite action.

My the drag-to-sort works fine, and the JS function I call on complete
re-calculates the positions just fine, but even if I don't specify a
value for the URL it still makes the roundtrip to the server.

How can I prevent this?

<%= sortable_element :line_items, :tag => 'div', :ghosting =>
true, :complete => "sortLineItems()" %>

I'm not sure why an asynchronous roundtrip to the server is not quick
and light. Even if the server takes a while to respond, the user
doesn't have to wait for that response, and in fact shouldn't even
realize anything is still pending.

It take about 1 second locally -- but there is no reason for it to make the
round trip. Calling my JS method using oncomplete means that the re-sort
doesn't happen until after the roundtrip.

I'm doing a render :nothing on the server. With Mongrel in a production env
this would probably be faster -- I just don't want to make the trip for No