Listens for when a drag movement has started on a mousedown. If you listen to this, the mousedown's default event (preventing text selection) is not prevented. You are responsible for calling it if you want it (you probably do).
Why might you not want it?
You might want it if you want to allow text selection on element within the drag element. Typically these are input elements.
Drag events are covered in more detail in jQuery.Drag.
$(".handles").live("dragdown", function(ev, drag){})