gistfile1.js

YUI({
  modules: {
    'module1': {
      path: 'path/to/module1.js',
      requires: ['modules', 'this', 'requires']
    }
  }
}).use('node', function(Y) {

//module1 is not loaded

  Y.use('module1', function(Y) {
    //module 1 has been fetched from the server and then 
    //loaded into the parent sandbox then this callback is called
  })  ;

//module1 is not available until the above use returns
  
});