jQuery.Class.prototype.init  function     

If an init method is provided, it gets called when a new instance is created. Init gets called after setup, typically with the same arguments passed to the Class constructor: ( new Class( arguments ... )).

$.Class("MyClass",
{
   init: function( val ) {
      this.val = val;
   }
})
var mc = new MyClass(1)
mc.val //-> 1

Setup is able to modify the arguments passed to init. Read about it there.